kung PCI-E ang interface ng videocard mo, try to find a nice but not to pricey Videocard

ex.
Nvidia 7300GT 512Mb DDR2 2130pesos lang sa pc express
ATI 1650 pro 512Mb DDR2 2130pesos lang sa pc express
ok to if your a midrange gamer not running on full resolution

but if your a hardcore gamer then you should get more expensive cards than the list above

like
Nvidia 8500GT 1Gb DDR2 3220 sa pc express
ATI HD2600pro 512Mb DDR2 3170 sa pc express


but if your a hardcore to the max gamer you should buy these cards but i tell you it will really cost you so much

Nvidia 9600GT 512Mb DDR3 8300 sa pc express
ATI HD3870 512Mb DDR3 10350pesos sa pc express
ATI HD3870 1Gb DDR3 20230 sa pc express

but in my experience you must consider the overall performance of the pc..
processor, memory, Hdd, PSU are some big factors affecting the performance of a system

here are my pc spec
Intel Core2Duo e4400 2.0 limited
2GB 667Mhz Kingston DDR2 (1Gbx2) Dual channel
160GB 7200 WD HDD
550Watts Enermax PSU
7300GT 256MB 128Bit DDR2 videocard
ECS i945GCT 1333mhz Mobo

and Manage to play Unreal tournament on it