There's really no need to spend even more than $100 on an SDR motherboard for an Athlon 900. Even the best SDR motherboards for Socket A fall well below the $100 mark at online retailers. My personal recommendations?
Asus A7V133 - around $100 (133-C model with no sound or RAID)
Abit KT7A - around $90 (no sound or RAID)
Asus A7A133 - $80
IWill KD266 - $80
All of these motherboard provide good tweaking options - that is, FSB speeds in 1 MHz increments and core voltage for the CPU up to 1.85v. The A7V and KT7A have the VIA KT133A chipset. Although relatively fast, this chipset had its share of problems, especially with the 686B southbridge. The A7A and KD266 both have the ALi Magik chipset. It's quite a bit slower than the KT133A chipset, but motherboards with it are cheaper, and compatibility issues are all but nonexistant. My dad has a KD266, and we have yet to see a BSOD with this motherboard.
Good luck with your shopping, and make sure to read up and ask questions before buying stuff; it can save you hassle in the long run.