Okay, I would say look into getting a full gig of ram and a geforce 8600GTS.
Why a Geforce 8600GTS? Well because the 8600 GT isn't that much of a upgrade from a 7600. The 8600GTS has Direct X 10, in case you ever get vista.
Also, the 8600GTS as good as the 7900, in some ways better. The 8600, though 40 bucks cheaper, isn't as good as it should be. Furthermore, the 8600 isn't as bad on your power supply as the 7900 and the radion 1950.
Check out these links.
http://www.tomshardware.com/2007/09/05/the_best_gaming_video_cards_for_the_money/page3.html
http://www23.tomshardware.com/graphics_2007.html
Don't be afraid to check other games on that chart.
If you check out those carts you'd see that the 8600 is quite a performance increase for only 40 more bucks. Also, look into getting a Atlhon 3500+ or better, they can come in 939 socket, and will play any game out, and any game coming out within a year, assuming your vid card and ram can handle it.
Be sure you get 184 slot ram. Also, check your ram speed before buying it. If you can. Because if you mix different speed rams things can get funny. If your not to sure, its not that big of a deal, you may just get artifacts from time to time.
Newegg.com sells the 8600GTS for 150ish and 184 ddr ram for 40ish per 512. Make sure you get 2 sticks, not one. DDR ram works best when paired with another stick of its type. If not, it runs slower. Right now, your PC should have 2 ram slots filled, each 256 ddr. If not, then you have 1 512, in which case you should buy a nother 512 stick. (I'd say get a 400 Mhz, any higher might not work right with your mobo). Also, Amd Athlon 3500+ 939 socket go for about 50 bucks or so, maybe less now.
If you can, and only if you can. I would say get a new PC. Why? Well, your PC uses 939 socket, and will soon be out dated. It supports the older ram types, which will become a pain to replace soon. Most of your parts can be put into a new pc, and will not have to pay that much to build a new pc. Look around on newegg, and you'll see its not that much for just a mobo/processor/power supply/ram (I suggest getting one that is 450 or higher). The rest can be transfered over. A new video card would come at a price, but you can get the vid card now and get the other new stuff later, but if you do that be sure to get a vid card that is worth it.
If you can't afford the 8600, then don't be scared to buy an 7600. 7600 can play most games out still, just not sexy like.
