Well if you need to keep it cheap, you'll be able to get a cheaper Duron system than a Celeron system.
First answer some questions:
1. What is this going to be used for? Gaming? Web surfing? 3D modeling?
2. Are you willing to spend a little extra cash for quality and compatibility right off the bat (in regards to your MB), or are you willing to deal with a quirky board to save some cash?
- El cheapo system (1GHz is now the base configuration IMO so...):
CPU: 1GHz AMD Duron ("Morgan" core): $40 (before s&h)
MB: ECS K7S5A (SiS 735 chipset): $51 (before s&h)
- Cheap but quality board:
ASUS A7N266-C (nForce 415-D chipset): $109 (before s&h)
Generally speaking, ASUS, MSI, Abit, Gigabyte, and Tyan make the most high quality mainboards. Up and coming mainboard makers include Epox, Shuttle, Soltek, and Iwill among others (Iwill is still pretty high quality though).
Obviously you?re not going to get the most high-quality board for a ridiculously low price of $51, so that's why I asked question # 2.