In this case, its either HD 5770 or GTS 450. If you want DX 11, then its better you go for these two. On whole , best you go for the HD 5770. Its faster than the 450 GTS in most cases.
But I doubt whether the CPU would bottleneck it. Also, its best you go for a better PSU as well I suppose.
The GTX 260 is good as well, but it doesn't support DX11, only DX 10.1.http://www.tomshardware.com/forum/291910-33-5770-275gtx-e5200
Also, what OS do you have ? 2 GB of RAM, wont do, you'll need 4 GB. if its a 64 bit its okay, but if you have a 32 bit OS, not okay.