Goi nailed it perfectly.  For me it took 6 months (ugh) to build my AMD Athlon 650 system because I only purchased parts if I found a deal.
As da loser suggests check out the anandtech hot deals forum there's always tons of hot deals on RAM, HD's, cases, motherboards, mice, keyboards, etc.
Here's how much my system costs me:
$54.90 Antec KS282
$136.00 AMD Athlon 650 MHz CPU
$95.00 Asus K7V motherboard
$79.00 VooDoo3000 AGP
$9.00 Memorex 48x CD-ROM
$109.00 Maxtor Diamond Max Plus 30GB (7200 RPM) hard drive
$81.00 128MB PC100 Kingston ValueRAM
$9.00 Logitech Internet keyboard
$5.00 Logitech wheelmouse man
$17.00 SONY 3.5" Floppy drive
$159.00 17" ViewSonic E771 flat CRT monitor
$193.50 Plextor 12x/10x/32x
-------------------------------------------------
$947.40
Currently you can buy 128MB PC100 or PC133 Kingston ValueRAM for only 
$49.95 after rebate with free shipping.
There also a possibility of getting a 
Maxtor 40GB 7200 RPM hard drive for $119
Of course I turned around and sold this PC to my roomate for $947.40 (the price I paid) and bought a loaded Dell Dimension 4100 for $2,000 with 19" FD Trinitron monitor.
I decided to buy the Dell because I didn't want to take the time again to "wait" for deals to build a faster system plus I got this monitor, and lots of freebies with my Dell system.  Right now you can get 
20% off any Dell system with free shipping.  Awesome deal!