The Q9650 is a beefy processor, but it's not worth the $300 premium IMO (and this is coming from a Q9650 owner).
If you really want to extend the life of your 775 system for a few years, I would look for a good deal on a used Q9450 or Q9550 on eBay and overclock it. By good deal, I mean don't spend over $150-170.
I upgraded to a Q9650 from a first gen E6300 and the boost was phenomenal. That being said, I use software that uses more than 2 cores so it was worth it for me. Yes, games were improved... but not by as much as a video card upgrade would bring.