Debian :: Turn On Graphics Acceleration Again Without Having The Screen Flashing On And Off?

I initially installed Debian Lenny on my system but couldn't get sound to work so I upgraded to squeeze hoping that the newer kernel would somehow fix that. Unfortunately, the update caused the screen to flash on and off starting at the login screen - I'd get a split second with the screen on and then it would go off for a second or two. The boot screen still allows me to boot from Lenny's linux kernel, and that still works. I also had this problem when I installed ubuntu on this computer so after some searching around I remembered that adding radeon.modeset=0 as a boot option fixed this. Now the screen no longer flashes, but everything is really slow - I think that disabled graphics acceleration? After some searching around it seems that these problems are probably caused by something called KMS in newer linux kernels but I don't really know.

Anyway, does anyone have any ideas as to how I could turn on graphics acceleration again without having the screen flashing on and off? Or suggestions for making sound work? I think in the past (on ubuntu) I ended up having to enable the proprietary graphics driver, but on the other linux distros i've tried I used proprietary graphics and flash and my computer got slower and slower and started to crash, so I'd rather stick to the open source driver if possible - i don't need really need 3d acceleration. Stability is the reason I switched to debian. My computer is running Debian squeeze amd64, intel core 2 duo e8400, ati radeon 4830, 4 gigs of ram. relevant results from lspci: 00:1b.0 Audio device: Intel Corporation 82801JI (ICH10 Family) HD Audio Controller. 01:00.0 VGA compatible controller: ATI Technologies Inc RV770 LE [Radeon HD 4800 Series] 01:00.1 Audio device: ATI Technologies Inc HD48x0 audio maybe part of the audio problem is that there are two audio devices listed?