1) Stick with Nvidia graphics. You want a 7800/7900, 8600/8800, 9600+
ANYTHING LESS WILL S-U-C-K AT 3D, unless your SCREEN is low-resolution. (We have all the variations at the office, and everyone complains that theirs isn't as fast as my 7900 GO, and lags and jitters in AC'ad 3d on their ATI's, except on their newest, fastest desktop 3700+ burning hot GPU's...it's smooth as butter on my Gf... WAY nicer than even on the older QUADROS by the way!!!! x1700 or older... the 7900 is much nicer in AC- how did they pull that off? Who knows.)
I.E.
The bigger the LCD RESOLUTION (not size) the more powerful your GPU needs to be. I've got a 1920x1200 LCD (17" in my case) and an older 7900 GO 512mb ddr2. It completely rocks, (it scores less than the lowest of the choices above, in benchmarks, by the way!!!) except when I have like 30 high-poly trees, and a 10 story building with a lot of curved objects inside and transparent (non-opaque) windows (so the interior GSM objects all get GPU rendering time as I move around the model- remember this if you go with a lower-GPU! hide those sinks and faucets and tubs and toilets while in 3d fly-mode, and make the windows opaque).
2) I don't see much difference at all between my T7200 C2D and my Q6600 desktop except in rendering times- the Quad is a tad faster (2:00s vs 2:30 minutes maybe?)
3) RAM... 4gb is good. 2 sticks in Dual is best. If you do a LOT of custom textures and use your machine for other things (heavy multi-tasking vs. one work app at a time, with maybe music on the headphones or a video in the corner) then you'll need more, but rarely do I touch my RAM, and I don't use a pagefile, so it's ALL LOADED into the 4gb I have- and I do heavy multi-tasking (so really 2-3gb will probably be fine for most people, even most architects- 98% of us)...
4) RAM SPEED ...Hmmm... Honestly the difference between DDR-400 and DDR2-800, or DDR3-1200, is negligible. YOU WON'T SEE IT OR FEEL IT. you might notice 3 seconds difference in a 3 minute project or render, but that's about it- unless you burn DVD's, watch a movie while ripping a 2nd DVD and rendering in Vray all while working in AC and 3dsMax and doubling up your laptop as a file server (yeah I've done it, a lot)... Then things might bog down. But otherwise, simply good memory, not expensive. Good means warrantied.
5) HD speed.
Ok, I'm an old overclocker, and I used to run crazy gear (specialized liquid cooling systems, you name it)...I stopped using 7200 RPM drives in laptops right after they began making SataII drives- simply they KILL battery life vs. a 5400 RPM and you WILL NOT NOTICE it in anything except accessing a giant *.wav file or huge photoshop PSD or a 800+mb movie. And then you'll only notice it for a few seconds. Like 15. Meanwhile that battery will never last long and you'll notice that ALL THE TIME. (the same holds true for the newer video cards- or high-clocked ones. And 8600 might be the jem out there right now...if you can find one)
6) Wireless... Well, these can be put in, along with Bluetooth chips, into about EVERY SINGLE laptop out there. If not, you can get an almost flush-profile card for a USB port that works just as well. Unless you have specific reasons to get ABG, all you really need is G- that's what everyone uses, including all the hot spots in public places. N is useless for now, and unless your using your laptop to P2P movies and huge 8gb files, N is useless for work. Stick with just G, or a simple USB ABG plug= Bluetooth the same (get 2.0 though with whatever you decide on bluetooth).
7) KEYBOARD-!!!!
I can't stress this enough. I got an XPS-1710 because I got a deal on it, but it's missing the numpad. BIG MISTAKE. Awesome machine, but I have to use a plugin one if I need speed. That's one more thing to have to carry. ALL CAd apps make tons of use of the numpad for accuracy and shortcuts. The more proficient you are, the more you'll use it. If you're getting a 15.4" laptop, check the FN-numpad situation and see how you'd feel about having to toggle it 500 times a day- or 100 times and hour. Make sure it's comfortable to do with ONE HAND.
Most laptops that are mid-range graphics from the last 2 years will work with ArchiCAD and CAD apps. Most users will benefit from BASING their choice on GPU/Graphics card FIRST and LCD second. MOST CPU's can be upgraded (though it's a tad pricey to do AFTER the fact), but any C2D will work great at close or over 2ghz. If all you do it one thing at a time, even a Centrino Solo Core will be fine as well.
After all that, base it on how it feels and the build quality, if you can hold and handle the machine. Macbooks rock because they have those nice Asus machined cases (you can get the macbook without mac internals, if you love the case, though the silver version is exclusive for Apple only- gunmetal and light grey are the PC versions). Dell makes really decent machines, and some of HP's are very nice too. Alienware sucks -all hype and no difference from anything except in benchmarking and games and bragging rights (mine can do your 5 minute task in 4.8 minutes dude!)...
(...I'll simply learn how to do the human part faster with keyboard shortcuts and I'll still be done faster and be at the pub, while you're paying off that beast, m'kay?)...
Macs are nice too- but they tend to run hot except the newest ones (still hotter than PC's, sorry applefans! I've got both and handled a ton of both), which sucks for lap use, without a thick book. Ugh. Burnt Knees anyone???
If I had the choice again, right this week, I'd opt for a Dell XPS 1530, with a C2D 45nm cpu, the fastest video they include (8600/9600?) and the nicest screen. I'd get the minimal RAM and I'd skip the DVD burner and just get a DVD-ROM (slot-load maybe?) drive, and I'd put in my own 4gb memory, huge 500gb 5400rpm 5-yr Western Digital warranty HD (the original would go in a USB/firewire case) and my own ABG/BT chip ($20 ebay). I'd load it with a nice 4 year coffee-all-over-it-replaced-the-next-day warranty and go catch a ball game with the rest or save for a replacement battery in 12 months (when they go off warranty and fail a week later, per murhpy's law).
Yeah. Especially if they made it in pink. LOL!
[edit] I think you can get one of those 1530's for like
$1500 loaded certified new... that's 1.2 the best of the above systems in the posts above, and no real noticeable difference in speed unless you plan on making professional 3d rendering movies this year, all by yourself on your laptop (good luck)!!!!