I have a Dell Dimension 9100 P4 630 3.0GHz and yes I know, what was I thinking? Well over the last 3 years it has been fairly adequate. I do a lot of online gaming and CAD/Max work. Financially I can't build the computer I want but I would like to max what I can out of this current setup.
After doing a fair bit of reseach, checking forums, and company websites I have come to the following conclusion of what I can/may need:
4GB 4X1GB PC2-6400 DDR2-800
Intel Pentium E6600 Dual Core or an Intel Pentium D 930 ( if they are both supported i'd go with the E6600)
XFX Radeon HD 4670 750MHz 1GB 800MHz DDR2 or
XFX Radeon HD 4650 600MHz 1GB 1GHz DDR2 or
XFX Radeon HD 4650 600MHz 512MB 800MHz DDR2 or
4 gigs of PC6400 will work ( 8 with a 64 bit OS ), look for ram rated for 1.8v.
No Core 2 Duo cpu's will work on your board.
The board will support a 1 gig video card, but low end cards really can't take advantage of that amount of ram.
There's no reason I know of why your current cpu would keep you from going to Win7.