I would consider the newer Core i7/i5 series from Intel. You should check these out as well. They have quad cores and some of them have hyperthreading technology which means you can run two threads simultaniously (not sure that's written correctly
) on one core and it actually makes it into a 8-core cpu.
They come however at higher prices but they are worth it as it is newer technology
Examples with newegg prices (all quad cores):
i7 930 2.8ghz: 289$ (hyperthreading)
i7 860 2.8ghz: 289$ (hyperthreading)
i5 760 2.8ghz: 209$ (no ht)