It's all about having an even balance between components in your box. I just recently built a new computer with a 64 bit OS and its everything I've ever wanted.
Intel core i7 920 (quad core overclocks to over 4.3 ghz without much heat)
12 gigs triple channel DDR3 (lots of memory takes the load off the hard drive when it comes to running samples and processing online effects)
2 - 750 gig, 7200rpm hard drives (I will soon be adding more HD's, but keeping samples and software separate from the recording destination drive will help keep the hard drive from having to jump around)
GeForce GTX 275 with 2 gigs of video memory (this just simply removes the graphic rendering load off the CPU)
A 750 watt power supply to handle all the power needs for the processor and other components. Then all the regular stuff like tons of cooling fans and a DVD burner.
I currently dual boot windows vista 64 Ultimate (i also use it for high end graphic design and gaming) and I couldn't be more happy with the performance of this system. I run tons of samples, record multiple audio tracks at once and have yet to hear/see any latency issues. I run Sonar 7 64bit with
a PreSonus Firestudio Project through firewire and its awesome to have a powerhouse computer that has yet to use anywhere near its processing capacity when working with large project files.
This cost me around $1700 after all the parts were shipped but to me it was well worth the benefits. I can power on my computer and start recording a track at 24-bit/96k sample rate in less than 3 minutes without ever worrying weather my system can keep up with it. Since I build computers as a side business I can tell you building your own (if you can) can save you lots of money in the long run.