Sorry, but you need to give much more information, like what type of cpu you want, amd or intel. What socket type? Intel can be socket 478 or 775(aka"T"). Amd can be socket A, 754, 940, or 939.
What do you want to do with this computer? Will this be just a motherboard replacement or a new sytem build?
$200-300 is a lot of money, many motherboards are about $100(usd) or less, with the performance boards reaching $200+. With $300 you can get a decent cpu and a motherboard.(like an amd 939 athlon 64 3000+ is about $150, and a 939 motherboard based on nforce 4 ultra would be about $85-150).