Decent GPUs for a 300W PSU

Hello, im looking for a graphics card that can play high-def games without the huge amount of lag im getting. Im currently using Intel Integrated Graphics with 1024mb dedicated VRAM as the new PC i bought didn't come with its own graphics card. I can play certain games like Left 4 dead 2 or Portal 2 on medium graphics, but when i start up GTA IV with everything set to the lowest the game is still running really slow. Not quite sure what to go for as i only have a 300W PSU. Any suggestions would be greatly appreciated, thank you (:

PC specs:
Windows 7 Home Premium (x64) Service Pack 1
3.40 gigahertz Intel Core i3-2130
Board: ASUSTeK COMPUTER INC. P8H61-MX R2.0 Rev X.0x
7110 Megabytes Usable Installed Memory (8GB Minus dedicated VRAM)
WDC WD10EZEX-00RKKA0 [Hard drive] (1000.20 GB)
Intel(R) HD Graphics [Display adapter]
300W PSU