The best processor for that motherboard is going to be the Intel Core2Extreme QX9650. A more plentiful and nearly as good processor is going to be the Intel Core2Quad Q9650.
Probably the best you can do is Core 2 Quad Q9*** series or even Core 2 Extreme. Although, considering these chips are out of production you're going to have to buy used from some place like eBay.
The best processor for that motherboard is going to be the Intel Core2Extreme QX9650. A more plentiful and nearly as good processor is going to be the Intel Core2Quad Q9650.