First you need to know if you have an AGP or PCI-Express graphcis slot on your motherboard. Once you find out then you will know what kind of graphics card options you have. AGP cards are slower and cost more now days.
PCI-Express:
ATI Radeon HD3870 w/ 512MB of video memory
ATI Radeon HD3850 w/ 512MB of video memory
http://www.newegg.com/Product/Produc...82E16814103051
Those are your two best choises for under $200, the 3870 might be slightly over $200 depending on where you look. The Nvidia GeForce 8800GT w/ 512MB of memory is a bit better than the 3870, but costs around $240. If you can afford it and want great performance for the money, get an 8800GT.
You can get a 3850 for $200 AFTER tax and shipping so that might be what you want to do. Plus the 3850 seems to have one of the best price to performance rations out of all the cards on the market.
AGP:
ATI Radeon 2600XT
http://www.newegg.com/Product/Produc...82E16814102718
ATi Radeon 2600PRO (best performance to cost ratio)
http://www.newegg.com/Product/Produc...82E16814102709
Nvidia 7600GS
http://www.newegg.com/Product/Produc...82E16814130076
Not too many good cards for AGP anymore. None of them seem to have a 256-bit memory interface. The only reason I recommend the ATI cards over the Nvidia for AGP is because the ATI cards have newer tech in them that will work better in the future. Besides, image quality is better with ATI cards than Nvidia.
*EDIT* oh, and judging from the look of your current card, you more than likely have an AGP slot. Building a brand new computer with a 3850, a nice dual core processor, and 4GB of good RAM only costs $800 now days. You might think about saving your money and upgrading to a whole new computer with the latest tech in it instead of spending so much on upgrading an old computer that wont be able to play games in a year or two.