No you cannot. Don't despair, though... the Core-based processors are faster then Netburst and Athlon processors, even in single-threaded applications. The only thing I know of that has its own thread in Freespace SCP is audio--which would be relatively simple compared to separating other parts (such as AI, collisions, and a bunch of other stuff, so i've heard).
I'd say you get at least a Core 2 Duo E6550--that processor runs at 2.33GHz on FSB1333 (333MHz). Be aware that, if you do that, you'll need DDR2 667 (PC2 5300) or faster RAM, such as DDR2 800, or DDR3 1333 or faster. DDR3 is expensive ($200/gigabyte, I think) and offers little benefit right now (especially compared to the under-$40/gigabyte DDR2).