Anandtech has this guide to Value Systems that you might find helpful.
This is my idea of a low-cost system with good overclocking potential:
Duron 600 or 650, late week cips have good potential to reach 900 - 1000, @$60.
GlobalWin FOP32 heatsink/fan @$22.00
Abit KT7 or Epox8KTA+ @$125.
128MB PC133 CAS2; Mushkin Hi Performance Rev 1.5 @$89. or Crucial @$92.
Guillemot Hercules Prophet MX @$135.
Sound Blaster Live Value @$45.
Cambridge PCWorks speakers @70.
Antec SX830 full tower case with 300W power supply @$85.
(or Antec KS282 mid-tower case with 300W power supply @$70)
Case fans @$13
IBM Deskstar 75GXP 20GB @$125
Generic floppy @$10.
I would go with a CD-RW instead of CD-ROM/DVD, you'll get a lot more use out of it even though it costs a little more. An HP 9310i can be had for @$150 after the new HP $50. rebate.
Total: @$930 + monitor and shipping
Good price search sites are:
IBuyer
PriceGrabber
PriceScan
Of course you can check out vendors at:
ResellerRatings
Just some ideas, hope they help.