|Well for starters you should upgrade the Ram to at least a 1gb of DDR 400 ram if you haven't donw it already. 256mb of DDR 400 that it comes with is just too weak for anything now of days.|
Kingston ValueRAM 1GB (2 x 512MB) 184-Pin DDR SDRAM DDR 400 (PC 3200)
As for graphics cards You have an AGP 8x slot. Any AGP card will work with it, but it all depends upon how good your processor and power supply is as well. Considering the processor is a budget processor and considering the power supply prob is a basic power supply in terms of power. I wouldn't go beyond anything ATI Radeon 3650 and that maybe even pushing it, but considering the options for AGP graphic cards are getting smaller and smaller a card like the ATI Radeon 3650 is the only real option left that can give you performance and value.
SAPPHIRE 100258L Radeon HD 3650 512MB 128-bit GDDR2 AGP 4X/8X HDCP Ready Video Card
Those two upgrades right there should help a lot in boosting the video performance. Now the prices may seem a bit high but you are buying discontinued hard ware platforms. DDR1 and AGP are no longer being made so that's why the prices will cost more if not a lot more then say DDR2 or DDR3 and PCI-Ex hard ware platforms
Iron Sharpens Iron.