Get a machine with as many cores as possible as lightworks (and artlantis?) will use them all. Two quadcores would be good at the moment. Extra processor speed will help AC run quicker generally, but extra cores will make rendering quicker.
Is it possible to get XP specified, as ArchiCAD still doesn't quite work properly with Vista.
One of the only reasons to want to use a 64bit operating system is to be able to use any extra RAM the system has. Specify 8gb or you wont see much difference over 3.2gb that a 32bit system will be able to use.
The graphics card is pretty good though. It will make navigating in the 3d window nice and smooth!
The system you specified will run AC well I expect, but you may have problems with vista, and any extra cores will make it more future proof.
Hope thats helpful in some way!