Hm, with only $50, I think the only reasonable upgrade would be RAM. If you really want to get better graphics for gaming, etc, you'll have to dish out a little more.
EDIT: Just realized you've got a Nvidia 6 series graphics card. Hm...maybe a 7 series would be a worthy upgrade in your case if you find a good one off of newegg.com with a nice rebate. I don't see the RAM being MUCH of a problem since I used my sony with 1 gig of ram for gaming and it was perfect.
Intel Q6600 @ 3204Mhz ~ Tuniq Tower 120 ~Corsair XMS2 5GB ~ WD VelociRaptor 150GB Main ~ WD 1TB Black ~ EVGA 8800GTS 640MB ~ 7 Home Premium (64-Bit)