So I'm thinking of upgrading my PC a little, but I'm not sure what card to get or if the money I'm willing to spend on it is even worth spending. Any advice would be appreciated

I was thinking about getting something like the ASUS ENGTX560 DC/2DI/1GD5 (what a crazy name!) or maybe a Radeon HD 6950? I have no idea what to buy, but it shouldn't cost more than the ASUS card, and unless an AMD card is a lot faster I'd rather stick with Nvidia, as I've had way too many problems with the AMD drivers (especially in linux).

All of my current hardware specs:
CPU: Intel Core I5-2500k
RAM: 2 x 2 GB Corsair XMS3 - PC3-12800
GPU: Gainward GTX 460GB Golden Sample - Goes Like Hell <--
PSU: Corsair CMPSU-750HXEU
Motherboard: ASUS SABERTOOTH P67 Sandy Bridge B3
Misc: Noctua NH-D14, OCZ Vertex 3 SSD, NZXT Panzerbox*, Microsoft Sidewinder X6, Logitech G9x

* Notice the Panzerbox. The card cannot be a huge card. The current one can only just fit in there.