I would stretch the budget a bit and put the 6850 with the i5-2400. The 2400 is only 10% slower than the 2500, and still faster than the A8-3850. You will be very pleased with your purchase. The 550 cards are slower than the 6850.
Yes, the 560 Ti is a huge leap over 550 /6850 .... it's a very popular proce point and both AMD and nVidia have dozens of cards in this price segment, all slightly different. The factory OC'd 6950's and 560 Ti's are the way to go.
Guru3D uses the following games in their test suite, COD-MW, Bad Company 2, Dirt 2, Far Cry 2, Metro 2033, Dawn of Discovery, Crysis Warhead. Total fps (summing fps in each game @ 1920 x 1200) for the various options in parenthesis (single card / SL or CF) are tabulated below along with their cost in dollars per frame single card - CF or SLI:
Thanks for the answers. Just one more question, I'm looking at different options now so my list has changed a bit. The only difference is between processors as I would like to stick with the Ati HD 6850 (GTX560 way out of my budget)
FX - 4100
I realise i5 has the AMD beat so I ask by how much? It's a $100-150 difference and I'm thinking GPU is more important?