You can't partition the 750s, but if you use the ICH9R you can use Intels Matrix raid. I believe this allows you to use only 500GBs of the 750, allowing you to still use the other space that would have been wasted.
I don't know what your using this for, but I'm pretty sure your doing it wrong. Sell the two 500s, and either one or two of the 750s. Use that money to buy an SSD, which will be plenty fast. Use the other 750 for backups.