The nforce 680i does not support 45nm quad processors, so the Q9550 or any Q9xxx/Q8xxx processor upgrade is not an option.
The E8400 C0 can be used with a BIOS update, but I'm uncertain if the E0 revision is usable.
If your aiming towards a quad, the best you can get is the Q6600 which is hardly worth the cost.
+1 to the GTX 260 idea. Though IMO, it may be more worthwhile to buy an aftermarket cooler and overclock the processor instead of buying a new CPU.