I just looked this up, a 32-bit OS can actually access up to 4gb of RAM (2gb for kernel, 2gb for apps).  Sorry for the misinformation.

tomshardware.com has good reviews and advice, but you may have to dig around a bit in their site and do a lot of searching.  But, in my experience, it really helped me make a decision on what to buy depending on my budget.  So, if you have the time, or willing to put in the time, this is a good beginning.

I think a 32-bit system can only address 2 gigabytes of RAM.  It may register more, but because of the way the 32-bit system is structure, it won't be able to use more than 2 gigs or RAM.  If you want to stay under the $1000, you may want to stay away from the quad-cores for now because the cheaper versions aren't really worth it because they only give a slight performance advantage, and as of my experience, most programs don't really take advantage of multiple cores, and if they do, having more than a dual core won't speed up performace.

As for graphics, anything above an ATI 1600XT, or above an Nvidia 7950GT should be fine and within your budget (taking in the other parts, that is).  I think nvidia lowered their prices for the 8800gts for some reason.  Try to get one with 512MB, but definitely get one with at least 256MB of VRAM.

That's my advice, but definitely visit tomshardware.com and search their database.  I'm probably a bit outdated with some of the technology out there.  Good luck on your new machine.