I would suggest getting something faster than the core2duo 6600. It's a nice chip and all. But now we're nearing the time where they will be releasing a batch of new chips. Not just that but, probably a lot of new audio software too. I suggest going with the 6700 minimum (2.4ghz I think it is).
On chips, ghz don't really matter as much any more because of how smaller the die is becoming. AMD is loosing that battle. Intel chips are set at 65nm, and about to become 45nm. What this means is more transistors can fit on a regular sized die. Transistors mean more clock work per cycle. Which basically means a LOT faster. On top of that, Intel might be integrating the memory controllers onto the cpu, which will help bottlenecks because of the processors being faster than motherboards can handle. I suggest running at least 2gb of DDR2, and sata II drives. Of course everyone knows that computer technology changes every day. Not everyone knows to what level though. a 1TB HD will come out, and that might seem to everyday, but the explosion of HD space is what you look at. Sure another processor is coming out, but the tech that backs it up will prove that it's just not another processor. Something that can keep up with audio programs & the resources it taxes are needed. Some people on here are using Pentium II/III's with 512mb ram, 40gb HD's, and say they are fine. Throw in 10-15 effects, on 20 tracks, and 10 vst's, and their computer will crap out.
the 6600 chips are great but I rather get the 6700 or the extremes.
Grab yourself either of those chips or higher end chips. On AMD's side, if you prefer, grab the 4800+ it outperforms the lower AMD's, and the 5000+ without busting your pocket. Or if you can afford it you can get a FX-61 just to barely keep up with the intel 6600's...
For memory, choose Corsair, Kingston, OCZ, Patriot, Buffalo,
Good memory has good timings, check the timings on them. The lower the better of course. Like my timing on my pc3200's are 3-3-3-10. Pretty quick memory for the older ddr1 memory. Memory like this
http://www.newegg.com/Product/Product.asp?Item=N82E16820227118 is pretty quick for DDR2 667. They wont' go lower than 4-4-4-12 unless you pay a lot more $$ for them.... I'll take that back, Super Talent memory's timing is 4-4-3-8.
For motherboards. That's a world in it's own, there's hundreds of motherboards. I personally like Gigabyte, Asus, Abit, Epox, Evga, and DFI motherboards. Those will serve you well. Gigabyte has great service, for once I sent in a RMA for a customer, and got a brand new board back with no questions asked, and pretty quick too.
For cases, keep it quiet, but, don't skimp on cooling either, you rather have a session noisy than have an artist pay you $1,000 for a session only to have the whole thing crash because of heat problems. This one, I'll leave up to the board to help you out on, because I'll just isolate my case, so I can keep my thing at a chilled 25 degree's C.
Power supply. Nothing under 500w. Period. Good if it has more than 1 12v rail on it too. Keeps your system running stable and longer life.
Also, some people might tell you "pci is phasing out" don't listen to them. It's not going anywhere. Even Altitude909 needs more than what he stated, and he's not alone on that one, though I do got a Gigabyte SLI board, but with 3 PCI slots. Or 4 I can't remember.
and to end this, I'll like to note that these newer chips that have the 4mb cache, are shared, which is a lot better. The older chips, like the Pentium D's, and I dont' remember if the 6300 c2d too, were not.