I have this card on my laptop.
The open source driver xserver-xorg-video-radeon for this card is old in lenny and does not support 3d acceleration well.
If you want to use the open source driver at all costs, you have upgrade to Squeeze or Sid. Their version is almost the newest available and It works well (except for some trouble people are having with suspend and hibernate:
https://bugs.launchpad.net/ubuntu/+sour ... bug/305301 )
If you want to stay in Lenny until Squeeze is ready you'll have to use AMD's proprietary driver or the fglrx module. Unfortunately the version of the driver in Lenny will not work with kernels > 2.6.27 (I haven't actually tried 2.6.28...). It works very well with the mainline kernel in Lenny and very well with my customized vanilla 2.6.27 kernel. If you are using any of these two kernels, then follow method 1 here:
http://wiki.cchtml.com/index.php/Debian ... tion_GuideThat should do it.
The 2.6.27 kernel is actually pretty awesome. Perfect for Lenny as the matter of fact. Because it's a Long term support kernel release and most of its bugs have been dealt with upstream (actually they have released 39 patches since it first came out). It has improved my system a lot (especially b43 for my broadcom 4318 wireless chip).
There are a couple of pretty good howtos for kernel compiling in the forums. Good luck