I don't think you can buy the entire guts a Mac Pro, not unless the case they were in literally melted around them and the owner decided to sell them rather then get a new case. Figure the chances of that are pretty slim. You can get a faster processor for MP 1,1 (plenty of threads around here), but not the main board. Frankly, though, coming from a mini, you'll find a base MP 1,1 plenty fast. You'll be better far off spending the 300-500 hundred dollars that you'd spend a processor upgrade on more memory or on an SSD.