Another Need Advice on Upgrades!

I am starting to have some troubles with stuttering and low fps in a game I play and wanted to know what I should look to upgrade first, with about $200-300 in hand now.
Thank you in advance

Processor Intel Core 2 Duo E8400 @ 3.00GHz
Mobo - ASUSTeK P5N-D BIOS Revision 0502
Memory - 2x2gb DDR2 400MHz
GPU - NVIDIA GTX 560 1gb
PSU - THermaltake TR2 RX 750W

The last two were relatively recent replacements to this aging machine.
Hard Drive is 300gb nothing special i am running win 7 home 64 bit.

Thanks for your time and thoughts