Without going with the expensive extreme edition CPU the Q9650 is the best s775 CPU. The Q9550 is a great compromise for dollar to performance ratio.
If those are out of your price range then any Core2 duo or quad will be a great improvement over your current CPU. Just get the best you can afford.
Though you will likely need to update your BIOS for proper support for either one. Also a good idea to make sure the board supports your new CPU choice before purchasing. Though I can find the board still being sold at Newegg I cannot find it at Asus's site. You may need to shoot them an email to get a CPU support list and the latest BIOS.
Ultimate Edition Linux
AMD FX 8350
Asus Sabertooth 990FX R2.0
24GB GSkill/Corsair 1866MHz
EVGA GTX 970 SSC/ GTX 650 (PhysX)
SSD - OCZ V4 128GB/Kingston 120GB
HDD - 3TB/2x 750GB/500GB
CM HAF 932 Red
CM Hyper 212 EVO w/2x 120mm Red
Logitech X540 5.1
2X Acer 23" LED - Surround: 3840x1080
Microsoft Sidewinder x4 KB
"The significant problems we face cannot be solved at the same level of thinking we were at when we created them."
- Albert Einstein