It would seem that there really isn't much of a performance increase in the latest ATI Catalyst drivers and it may well mean that some people will not worrying about updating their VGA drivers this time around. Doom 3 sees a slight increase at the lower resolution as does Unreal Tournament 2004 but when it comes to synthetic tests there seems to be a slight performance drop.
The 4.9 BETA drivers are slower in just about all our tests except Doom 3 but when these BETA's were released by ATI, they did come with the name Doom Hotfix so we didn't expect to see much performance gains across the other tests - just the super hot new FPS. With that said though, we didn't expect them to be slower. These are still in BETA status so we can't really comment on them or judge ATI in anyway until the Canadian firm releases the official drivers next month.
The only reason you would upgrade to the latest 4.8 drivers from ATI is if you are playing a lot of Doom 3 at the moment - at 1024 X 768 there is definitely a performance increase and while the 4 or 5 FPS might not seem like much, when working out the percentage it can be as high as a 10% performance increase in the new shooter.
If you are not intending on playing a whole bunch of Doom 3, as far as pure performance numbers go, there should be no need in upgrading. Although as a general rule, it is always good to be running the latest drivers as there are also bug fixes in each and every release.