First of all you need to get rid of intel shitty onboard video. Then you can think about playing games. As I heard you can only play CS, NFS games up to most wanted and world of warcraft with your current intel onboard.
P.S- First don't be fool by the 1GB graphics cards are way better than 512MB cards logic. It's just marketing trick shit. Hope you understand what I mean.
Anyway I can suggest you few graphic cards available in SL market according to your current system such as Nvidia GT 240 < ATI 5670/ ATI 6670. Else you can use 9600GT< HD 4850/ 9800GT < 8800GT's but they are more than 4 years old and use more power and generate more heat than new generation cards.
Finally keep in mind that you can't go more than something like GTS 450 cos,
1. your CPU has not enough juice to handle mid end card better than GTS 450.
2. we don't know about your PSU. If you want to use a GTS 450 on your system you need an external 6 pin power cable to power up GPU. So I strongly suggest you to put a pic of the sticker on your PSU here. Then we can decide what to do. Otherwise there are option like HD 5670 which is very very easy to find in SL, uses very less power(dont need any external power and can be run on any cheap PSU but preforms better than 9600GT (Yeah! 9600GT also need an external 6 pin power cable as well.)
So I think now you can decide what to do. PM me if you want to know anything else.
හොද VGA එහම කියපල්ල 
- Graphic card 
