One question, what do you use your computer for?
Unless you say extensive 3D rendering, or compression of large video files, or some other massively CPU intensive task, it'* hugely not worth the money.
My AMD single core, 32 bit 2600+ is still an awesome computer. Albeit I also have a dual core 64 bit 3800+, AND a Core Duo laptop which is equivalent to about a 4400+ and honestly the times I notice the laptop being faster are rare.
99% of the population should be investing their CPU money into high end hard drives, that'* when you can genuinely say, "Wow, that money was worth it"
Instead spend $350 on some 10k or 15K ULTA160 SCSIs, stickm' in RAID 0 and notice a real difference. Or for simplicity just get some WD Raptor'*, less overhead cost than SCSI drives, not as fast, but still worthwhile.
Big difference between a 12ms and a 3.5ms seek time!
(Ok, so that was partly a rant over the new Core2s
and people being obsessed with them.)