Simp barebones is probably best. Then just pick up some RAM. You can save money by going AMD instead of Intel but I'm sure you figured that out already.
Just some general advice to follow, no brand/models:
I'd strive to find a motherboard with less on board features the better, honestly, as most are crappy. This is in regards mostly to video and audio on board. Some do have decent to good built in NIC's but do some HW to see if it's a repuatible/trouble free one. Anyways, most likely a PCI card will be a better solution (most definately in audio/video and probably in NIC/firewire/usb).
I can say I never have had problems with onboard USB/Firewire but never seem to get those damn on board audio/video slots to work.
http://www.pricewatch.com has the ability to search for certain Motherboard Combos, which you might find helpful in getting a good deal on CPU/Mobo together.
You'll see more of an impact in performance by upgrading the memory then the processor. What I mean is that if you are looking between a 1.5GHz processor w/512meg of RAM or a 2.0GHz w/128megs, the 1.5GHz processor computer would probably feel like a faster machine. So perhaps consider that when comparing close processors, save some of that money for more RAM later. Of course, this applies to those with the same FSB/cache/etc.
Ack, sorry for the length.. forgot something. Don't forget to check out reviews pertaining to the chipsets your looking at. There is some difference in terms of performance when you compare SiS chipsets to VIA--I've heard good things about the nForce2.