Seeing as how you are going to have to buy some new memory in addition to the board, it's going to cost you over $150. For that kind of money, you can get a Radeon 8500 or GF3 Ti200. Since you already have an A7V133 (Performs quite well) with 512MB of Crucial memory, I doubt you will gain any serious performance by going DDR. The 3D Prophet 2 Ultra to Ti200 upgrade will give you much more of a benefit. Of course, you probably won't take my advice. If you don't, I would suggest the Shuttle AK31A, EPoX 8KHA+, or Soltek SL-75DRV2. They are all stable and fast. For overclocking, I would choose between the EPoX and Soltek boards. For light overclocking or no overclocking, go for the Shuttle.
<< And also what's the diferance in AMD XP and AMD MP, and which one's better? >>
The only difference is the MP is tested to work in SMP configurations and the XP is not. However, a good number of XP's seem to work in dual CPU configurations anyway. There are no performance or architectural differences between the two.