I am one of those who says that if you do have an S478 motherboard, it's time to do a real upgrade.
And there just aren't many cost effective upgrades for LGA775. A Q9550 is what? Around $250 or occasionally $200 at microcenter. That will buy you an inexpensive AMD CPU and a pretty good motherboard. And you will have a much better system.
The only time upgrading a Core2 system might make sense is if you have one of the older, slower chips. In that case, one of the E6000 or E7000 Wolfdales may make sense - if the motherboard supports it.
Here's an earlier post that shows what I mean:http://www.tomshardware.co.uk/forum/290146-10-memory-core-e6400-ddr2-ddr2-1066-ddr3-1067-ddr2