This would be a better build. The 550ti isn't a great card. The 7770 would be a better performer. Don't get 2 x 2gb. You would be filling up your ram slots limiting you to 4gb. If you have the extra money definitely get 8gb.
You can actually get 8gb right now and still be under budget.
The only reason I decided to go with the GTX 550ti is because its a pci-e 2.0 and I read that the Pentium G860 only supports up to pci-e 2.0 however the HD 7770 is a pci-e 3.0 so would they be compatible together?