The Q9650 is the best processor and cheap, but the "best" is the Intel® Core™2 Extreme Processor QX9775. The biggest difference being a 1600Mhz bus instead of 1333Mhz and 200Mhz higher frequency and it runs hotter. Pair this with a ASUS MAXIMUS EXTREME LGA 775 Intel X38 ATX Intel Motherboard or the EVGA nForce 790i Ultra and you got the best of the best (as far as LGA 775 goes) That processor is super rare though and I could only find 1 of them going for $9999.99. So stick with the Q9650 and you won't regret it.
UPDATE: After some research I found out that the Xeon E5472
would be a better option over the Q9650. About the same performance or better and cheaper as well. Actually any of the socket 771 Xeon processors take you pick. You can do a pin mod (tape / sticker) to convert from the socket 771 to 775.
http://www.delidded.com/list-of-intel-lga-771-xeon-processors/