The CPU is fine.
For Ultra, I will assume you mean @ 1080p.
The 7850 will be a 30% improvement. Ideally, sell your 6850 (They go for £60-£80 used, £70 usually.) on Ebay.
Might even be possible to find a buyer on Toms. I'd buy it from you, but I've yet to receive my debit card transaction-thingy.