|For someone who works in a computer shop, you sure do ask a lot of questions.|
Why would you be worried about the 4GB barrier? The video card will have much more impact on gaming than the RAM...4GB is more than enough. Stick with XP 32-bit for best performance.
I believe you're an Intel guy so just get a decent Core 2 Duo, a board based on the P43 or P45 chipset, 4GB DDR2-800 & the best video card you can afford. Video cards tend to drop in price quickly so don't go overboard...what you pay $200 for today will be $100 (or less) in a couple of months. And make sure to get a damn good power supply.
You can always add more RAM & upgrade to a 64-bit OS at a later date if necssary but by then, you'll probably be wanting a Core i7, DDR3 & Windows 7.