I've been using Lenny for over a year now with an nVidia card, and it was always pretty straightforward to install the proprietary nVidia driver so long as you read up on how to do it first. A chap on the Debian user forum posted a howto which worked fine. The small difficulty is that you had to make use of the Sid (unstable) repository to get the nvidia-glx package which contains the driver.
Since then, the full set of nVidia packages have made it into the Lenny repository, and enabling the proprietary driver is a simple matter of installing three nvidia-* packages. So the short answer to your question is 'yes'.