If this is a prebuilt, the existing PSU will definitely impose limits, unless you upgrade it also.
If, for example, you don't even have a PCIE connector, or if you have only a 250W PSU, you'd be limited to a 4670. That's much better than an 8400GS, but still won't play the "latest games" on very high or even high settings.
@coozie, thx a lot for the advice.. i jus checked the specs for HD4670 and it looks lighter on the pocket and better in performance as well. IF necessary i can improve the psu if 9600 can give better performance... do let me know.. thx again