I upgraded the GPU to a 4890 just because it is a better card and still in your price range. The P55 will be able to use any graphics card that you plug into it. If you're stuck on ASUS, that would be the mobo I suggest, but you could also go with a Gigabyte that I like here which is only $5 more expensive and still has free shipping (in the states).
Also, do you need any other parts, such as a case or PSU? If you do things may need to be changed around a bit, but here's what you asked for, and I hope this helps.
Well, as for the graphic cards, it looks like the HD 4890 is beating the GTX 260 in most benchmarks as seen here but it seems that the 260 is definitely keeping up. They suggest buying a 4870, or going up to a GTX 275, both of which are not bad cards, but it's up to you.
As for cooling as PSU, these are two things that I don't have a whole lot of experience in. The best way to find out if your PSU is big enough is to use the Newegg PSU calculator. I'd also like to here some other suggestions for this system.