It basically works like this.
Faster the computer, the better.
I would look for and i5 or i7 processor. Not just for speed but they have really advanced battery life technology and I've seen an i7 laptop run for almost six hours running AC.
8gigs of ram or more.
A dedicated video card such as Nvidia or AMD with a gig of ram. Archicad fortunately doesn't need a Quadro or FireGL card so the consumer version of those cards will work. Tho for best image quality and stability, I would choose a professional card.
The most important part...
The monitor. Don't get too small of a screen if you plan to use you laptop's monitor for full time use. I would get at least 1600x900 but personally I won't get a monitor with resolution less than 1920x1200 (which are actually becoming rare). You can get a smaller screen but I would recommend making sure there are two video outputs so you can run dual external monitors with a keyboard and mouse when you need to do work and if you need to be mobile it's still a laptop you can just unplug and go. (I have learned most laptops will need and external fan if you do this as usually laptop cooling systems are designed to handle the load of just one little monitor on the video card and expanding the desktop like that will make the card run substantially hotter.)
Lots of hard drive space. SSD's will also provide advantages over diskdrives.
I like the pro lines from Dell, Fujitisu, and HP. HP is out of the market now so that's not so much an option although you might find some great discontinued model deals. There is the Lenovo Thinkpads and there are some very interesting options. Personally, if the computer company wasn't Chinese I would highly suggest them but all I can say is caveat emptor, you may need it. Mac is another option but now that Steve Jobs has left Apple I have a feeling Apple is going to decline, mostly in quality, as the new guy seems to be a bit of a bean counter and quality is the first thing to go as it's hard to see the value in the bottom line. There is Sony but I wouldn't buy them because they abuse the customers and I wouldn't trust them at all for anything right now but they do make good equipment, can't argue that.
If you want minimum. I would get an i3 (battery life considerations), 4gigs of ram, and Intel 4500 videocard, with a 1366x768 monitor. That will work but you will feel confined and limited and big/complex projects will make the computer slow.