It will be a big deal, Windows 7 adoption rate is huge, and Vista SP2 supports DX11.
There's a ton of games already out with DX11 just 6 months after it's introduction.
If I were you I would just buy a 5850 now, Fermi will be the top-end when it's released, and affect prices of the 5850/5870/5970.
The $100-200 range won't be affected for a while.
I think that the OP doesn't have that amount of budget for GPU?
Yes a 5850 would be alot better than a 4850 and 5770 but it's about $300