Well, I guess that just goes to show how "out of touch" I am with current mobo/processor technology. I had to dispense with keeping up my AutoCAD subscription after receiving the 2008 ACADMAP upgrade due to job loss and the economy. I hadn't upgraded my computer previous to that since about 2004-2005 so I'm working with a rather "dated" system that can't even load/run the 2010 trial version. I'm trying to find comparison specs and info online to help decide whcih motherboard/processor combo would be best to minimize my expenses and yet still be viable to invest in to work with current perhaps "Student"versions, of AutoCAD and Revit. I think a Quad-core AMD should be adequate but I'm not sure to what extent AutoCAD avails itself of multi-core productivity. The technology is getting rather complex and confusing when you start trying to decide where to cut corners to save money over the cost of a whole system. I'd like to have 16 GB of ram and a decent video card card also so I doubt that I'm going to able to keep expenses under $600-$800; perhaps a thousand to upgrade the power supply along with it all. Actually, the more I look at it, about the only things I probably won't have to buy are a case and hard drives. What recommendation(s) would you suggest for the best, objective, online reviews and feature comparisons that explain, in layman's terms all the nitty-gritty details of specs to be concerned about with regard for practical application usage of AutoCAD and 3D graphics?
If you want more than 4GB RAM, then the 32 vs 64bit Operating system question is answered as you can only use 4GB in a 32bit OS. As far as processors go, I can tell you that buying the "latest and greatest" is a poor investment in terms of cost vs performance. Currently, AutoCAD does not make use of multiple cores except for Rendering and Mental Ray, so you want weight your choice toward processor speed rather than number or cores. I have AutoCAD2011 on a couple of first generations dual core computers and they work very well, so any processor built in the last 2 years should be fine. Make sure you get dedicated graphics rather than using graphics integrated into the motherboard, other than that unless you're doing heavy 3D work, you don't need an expensive workstation class video card.
I would definitely upgrade your hard drive(s) if yours are as old as your build. The smallest SATA drives that are available now are probably far bigger than your current drives and probably go for ~ $50
Regarding 64bit OS's. Yes, you will almost certainly have some difficulty running/installing some of your old apps. The good news is that Win7 Pro (im not sure about Win7 home) allowes you to run as a XP virtual machine, so while it can be a bit of a nuisance, If you can't run something you need from the Win7 environment, you can switch to an XP virtual machine