See this page for a list of all supported processors. http://processormatch.intel.com/COMPDB/SearchResult.aspx?Boardname=d975xbx2 . At the top you will see the the Core 2 Quad and the Core 2 Extreme series. They will be the fastest. It all depends on how much you want to spend.
Thanks! That tells me exactly what I wanted to know. (And I was not able to find that page from the motherboard support page.)
Nice to know that what I have is not too far from the top (Q6600)