Agree with previous comment, though I purchased mine tailor made to my spec from MESH as I didn't have an old machine to start from and this was highly cost effective (as well as giving a guarantee).
Basic spec as follows, noting I decided not to touch Vista just yet and opted for XP Media Center Edition instead.
Motherboard: P5N32-SLI Premium
Processor: Intel Core Duo 6600 2.4Ghz
Memory: 2.0GB 667Mhz
HDD Capacity: 2x 320GB Seagate Barracuda ST3320620AS + external LaCie HDD
Graphics Card: NVIDIA GeForce 7600GT
Optical drives: 2x Sony 18x "all format" CD/DVD read/writers
The result is a wonderfully swift and capable machine on which the only real problem has been a crap Kworld TV USB tuner. I am replacing this with a Black Gold device. The Barracudas are superb discs being quiet in operation, reliable, and with very fast access times. I am just as happy with them as this review (which helped me choose them) would imply:
http://tinyurl.com/3565dy
I have recently got in to video authoring - where head movement and seek times can quickly become a bottleneck - so will also add two more HDDs giving four in total. This will then give me one for the o/s and application, one for video source, one for audio source, and one for authored output. As I have done already, these HDDs will be carefully partitioned to try and help keep performance up including to allow me to use them for storage as well as their "temporary file" function during authoring.
Finally, for the partitioning I use and am most happy with Acronis, for defragmenting I use Diskeeper Pro, and for crap cleaning (temp files, browsing history clutter, etc) I use the free utility of that name, CCleaner. Nearly a year on and now with loads of data and installed applications, the PC flies along as well as it always has.
Good luck.