Another GTX 460 is an enormous upgrade; I can vouch for it as I just did it myself. That said, the 6950 2gb would be a better upgrade provided that you are VRAM limited in the games you play. That should only apply if you game at 2560x1600, or else 1920x1200 with tons of AA.
If you're going to get anything other than another GTX 460, make sure it has at least 2gb, or else it's pointless IMO.