Sorry to tell you this, but you're out of luck. The LGA 775 socket (the pin pattern of the CPU) went obsolete a few years ago. You'll have a very hard time finding compatible processors. The q9650 was the fastest chip of that line, but you will pay 10X what the chip is worth because it's so hard to find now.
It's time for a full system upgrade. I know it may not look like CPU clock speeds have increased that much in the past 5 years, but trust me, they are much faster than what you're running.