Look at the sticky post above ( Favorite MotherBoard Manufacturer? ) for peoples preference.
I prefer Asus. I have installed them almost exclusively for customers, and have had only ONE warranty.
For $300 dollars your issues will be:
What components do I already have that will be compatible with a new Mobo.
1. CPU - socket, type and bus speed.
2. Memory - what do you have already, type and speed. (ex SDRAM PC133, DDR 2100, DDR2 6400)
3. Power supply, some new boards require a newer PS, 24 pin.
4. Video, if new board doesn't have it built on, is current video on board, AGP or PCIE.
Also if you are running Win XP and don't want to reinstall. Do some research for booting XP with new board. Usually if you uninstall all your components from the device manager, shut down, install new board. Windows will boot, find new devices and run fine.
The best option though would be save your data and reinstall.
Hope that's helpful in getting you started. Good luck.