Case, power supply and fan combos are best at Dealsonic.com IMO. They have a sale on most of their Skyhawk products and also have a lot of good stuff on clearance.

Motherboard - a lot of people will say go with something other than Asus. But I haven't had any issues so far with either the A7S333 on the home pc or the A7V8X-X on mine. The place to go would be newegg.com

Processor - you can get the 1.8 GHz Athlon XP 2100s real cheap now. If you want more processor power go nuts at newegg.com.

If you need a new video card the ATI Radeon 9000s 128MB DDR are cheap right now. Either newegg.com or dealsonic.com would be ok pricewise.

RAM - DDR 2700 is kinda pricery... but that depends on what you consider pricey. Check techbargains.com they have a current list of RAM prices. I have 1GB of DDR 2100 on mine and it does me just fine. Officemax has some sale going on right now on RAM, you can rip them off with $xx off $xx coupons too.