I'd second all above. I'm running a high end core quad (Q6600) and have no issues with performance but very limited for future upgrades with both that and DDR2 ram. Using 4 gigs of ram and would like more, you can never have enough
I would suggest getting the best motherboard and power supply you can afford and kitting it out with a cpu, ram and graphics card that gives the best price/performance for your remaining budget even if they are quite low spec as this will give you a fairly future proof system thats easy to upgrade later. I would put a high priority on the power supply as it can easily wipe out everything if it fails.
When you can afford to upgrade imho a top end cpu is worth getting, improvements have slowed down and you should get at least a few years from one. Graphics cards are still improving fast and there promises to be a whole new generation of computing as more is done with them (opencl, cuda etc). I would go for something slightly below top end and upgrade regularly.
Solid state drives seem impressive but I have no experience of them, worth checking out though. Conventional drives are cheap, if you use them instead get 3 and use software RAID 5, good performance and much lower risk of data loss (cant do this with windows :P ).
Good luck with it, no lack of experience on the forum if you need a hand