If it does hold you back a bit, it won't be much.
Here's a Sapphire Radeon HD 7970 3GB for $450
after $10 rebate
I wouldn't worry too much about your PhII at 3.7GHz holding that back too much, either.
Check out Ryan's OC Guide and post up any tweaking questions over at the AMD Over-Clocking
forum. Upping your IMC/NB from stock 2000MHz to 2400MHz+ can add quite a few frames if yah need them . . .