Sold my old 580's and bought some cheaper (paid $350 a piece, but sold my 580's for $400) Windforce 670's in SLI.. Gotta say, they overclock nicely and stay rather quiet..
Even if you could overclock your current CPU, it wouldn't make much difference seeing as you may get 200 to 300Mhz max over stock.. You would be better off dumping Vista and your 5400 rpm HDD and loading Win 7 on an SSD.. Now that...
Umm.. I'm thinking you're stuck at stock clocks.. You can try SetFSB or another software overlock program in windows. But, flashing the BIOS with a non-Compaq one is not recommended.