GTX260Graphics clock: 576Mhz
Memory Clock: 999 Mhz
Memory interface: 448-bit GDDR3 (this is older)
Memory: 896 Mb
GTX550TIGraphics clock: 900Mhz
Memory clock: 1025Mhz (4.1Gbps)
Memory interface: 192-bit GDDR5
Memory: 1024 Mb
HD6870Graphics clock: 900 Mhz
Memory clock: 1050Mhz
Memory interface: 256-bit GDDR5
Memory: 1024 Mb
HD7870Graphics clock: 1000Mhz
Memory Clock: 1200 Mhz (4.8 Gbps)
Memory interface: 256-bit GDDR5
Memory: 2048 Mb
Looking at this list the HD7870 comes out best. I would not go for the GTX260 at all since its already 4 years old (it came out in 2008). People often make the mistake that memory is what you need to look at. Like a 2GB card is better than 1GB card etc. Thats absolutely false. Its the memory, graphic and shader clock you need to look at and the memory type as well GDDR5 or DDR3 or GDDR3 etc etc. The memory of like 1GB or 2GB is just a buffer and nothing more.
If you want to know more or see which card comes out best. Just look for video card benchmarks on google and go from there.
Keep in mind that Nvidia cards also use PhysX. ATI cards do not. So usually Nvidia cards do run better.
Here is a website with huge lists of benchmarks:
http://www.videocardbenchmark.net/ Use it.
VC-MP: 
