At that price, you'll have to look to Semprons or Celerons. I'd go with the Semprons (socket 754) and stock cooling.
Stay with a basic board, say Soltek K8M800I-RL
Nothing overly expensive, but functional with onboard graphics.
Look for a good deal on hard drives. 80gig, if possible.
You can go with cheap memory, so long as it has been tested. Don't go with less than 256MB, 512 would be better if it will fit in the cost.
Plain vanilla case, but with a decent (if small) PSU. Your warranty would be worthless if the PSU fried it all. Some of the low end cases actually look fairly decent.