This may not be the answer you are looking for, but I thought I would throw it out there...
I got into the computer field building PC's/network support as a job and was a diehard "piece it together yourself" until a few recent experiences with bad components ordered for family/friends computers caused me to pull my hair out and spend way too much time on very simple things. My time is far more valuable these days.
I decided that my <gasp> Pentium2-300mhz wasn't cutting it anymore (I have a P-III laptop that I use primarily, which is also aging, ARGH!) and looked at component prices for a "decent" desktop/server.
My decision:
I bought a Dell PowerEdge SC 400 server for $400 after rebate (and a free Palm handheld you could hock on ebay for some $$).
It wasn't much more expensive than the bits and pieces (arguably cheaper since Dell gets stuff in larger quantity). You just need to pick out a good deal and not spend money with Dell getting upgrades. Just get a bare-bones box and let them "eat the losses" on the essential components since you don't pay for a memory upgrade that for the same money, you can buy the same amount of money and keep the RAM that isn't retained from upgrading.
Anyhow, the PE 400 SC (as I ordered it):
Pentium 2.4GHz @ 800Mhz
40GB IDE HDD
Floppy
CD-ROM
Keyboard/Mouse
Quiet case with nice ventilation
1 year warranty
No worrying about "is the ram bad, etc...etc..." was the important thing for me!
Downside: only 128MB of 400mhz ECC DDR RAM, 40GB HD (I have some 120/160GB drives I was using in a USB enclosure that got stuffed in there), no fancy schmantzy DVD-RW.
Solution:
Get more memory
Stuff some basic components in there to "soup it up"
As for prices on computer stuff, aside from pricewatch and pricegrabber, you might also check out bensbargains.net and other sites linked from there...
--Ashraf