hmm...they should, but last time i checked on the ATi website, there was a different tab for 4890 drivers
...you know when you select which drivers you want to download, there's a differnet tab for 4890 and 4800 series, if you get what i mean.
nonetheless, if the driver thing isn't an issue, both cards will run at the speed of the slower card, and yes you will get a pretty big increase in FPS, and your q9550 should be able to handle it. majority of games will work with crossfire these days. it was only when the 4800 cards were just released that there were crossfire issues. these issues are pretty much gone now.