You will want a mobo that supports CPU's that are 775, which is dual core, quad, etc... You will also want to get a new vid card, the 8800 gts is pretty inexpensive these days and will play most (if not all at this point) games. Oh, and get 4 gigs of ram is using XP (well, really 3 gigs, but hey, more the merrier) or 8 using Vista (not sure if vista supports that much though, someone correct me). If you want to go hardcore, for with a i7 board and chip, but that will cost you.
what are you willing to pay for the upgrade??
cause you sure need a lot of money cause you have to change the CPU which means that you have to change the MB which also means that your RAM wont work on your new MB so you'll need new RAM and of course you need a new VGA and that needs a new PSU so after all you'll have to change everything to be able to play today's games
but my motherboard can support alot of recent CPU's.
And im willing to spend maybe $300 on an upgrade.
Ummm hate to tell you this but the P4 is not even close to recent nor was it a very good gaming chip even when it was new. You have a 1st generation LGA 775 board that supports DDR memory and the P4 5xx and 6xx series. Also the 7600gs was weak even when it was new.
Wait untill you have more to spend and build something that will last. Dont throw away money trying to upgrade this system.
You need a new motherboard, CPU , RAM , Graphics card , Power supply and Hard drive. I really wouldnt even try to salvage any parts off that old thing if you really want a gaming system.
With a few hundred more dollars you can build a core2 or AMD based system that will smoke what you have now.