go for the same GPU but different model. I have 2 asus GPUs and wish i didn't, i'd get about 25% better output from a different branded 8600GT, bad spend by me. The motherboards are excellent though so don't panic over that.
As for price just have a look around other websites or, if you have any close by, nearby computer shops for better prices. If you have to mix and match between places for best prices it wont make a difference to your build, you will get the same stuff at better prices but from more than place.