Look at this page for supported processorshttp://www.intel.com/support/motherboards/desktop/d945gnt/sb/CS-026630.htm
and you will see that the highest you can go is a Pentium D 960. If you can find it cheap go for it. Depending on what games and resolutions you're gonna play you probably won't get a benefit out of anything faster than a Radeon 3870/4670, or an nVidia 9600GSO with your current CPU. With the dual core 960 you probably won't notice any improvement with GPUs beyond a Radeon 4850, but an 8800GT may prove a good fit.
Considering I don't see Pentium D960s for sale anymore you may have to scoure eBay for one. I don't think it's worth the Hassle, or the money, so in your case I'd just put in a Radeon 4670.