Graphics Card/PSU Compatibility Questions

Hey guys,

I have the XPS 8300 mode, with i7-2600 (3.4 GHz) and a GTX 560 Ti. I want to upgrade my graphics card and I know that the PSU is 460W so I don't mind if I have to upgrade it but what is the compatible graphics cards for my XPS 8300? Are these the only compatible ones?

KP8GM AMD Radeon HD5450
HWHRN AMD Radeon HD5670
GCJ42 AMD Radeon HD5770
2XTG4 AMD Radeon HD5870
HCVMH AMD Radeon HD6450
8F60V AMD Radeon HD6670
8PJF8 AMD Radeon HD6770
Y9XH7 AMD Radeon HD6870
4VDWW AMD Radeon HD6950
VH86X Nvidia G405
X78HM Nvidia GT420
WGP2G Nvidia GTX560

Because if so, what is more powerful? two GTX 560 Ti's in SLi or 2 HD6950 in Crossfire since I know you can have 2 Graphics Cards in these forums.

If I upgrade my PSU to say 600W, what if I get a GTX 670, would that be compatible since I was looking at that card?