5.6's work great, but if you use Driver Cleaner, it'll work so much better for upgrading. If you follow it's instructions exactly as they are written, this little program will make upgrading to new drivers so much easier.
I've been using 5.6's and the only game that's given me grief is GTA San Andreas. Randomly, it will throw the VPU Recover module on my card, and that worries me, but I think it's a driver issue, as no other game does this. That's about par for the course with Rockstar and it's iffy console to PC Ports.
Only downside to using the newest CAT's is in FSO, you dont get the nice glows on the ships unless you revert to driver version 4.4 if I recall, but there's so many optimizations for the new games out there that you should run the 5.6's to get the maximum benefit out of your card.
I'd reccomend for you to get, well when your warranty expires of course... to get an ATI Silencer VGA Chipset cooler, made by Arctic Cooling, cause these damn ATI Radeon's run HOT. My card, clocked at default settings, with a 9800 pro BIOS would idle at 68 degrees C, and top out damn near 80 degrees in anything that made it work hard. Since putting on the cooler (the thing is MASSIVE!) my card idles at 63 and tops out at 68 degrees Celcius. Yes, I cant believe it either but that's what ATITool reports and it queries the sensor chip directly... Since then I've made a very simple mod by flashing my card's BIOS from a 9800 Pro to an 9800 XT! Memory timings are much tighter and the card has given me a boost of 400+ points in Futuremark 05'.