To clarify things, the version currently on Debian lenny is 5.0.51, this DOES have the 4GB limit mentioned in the previous post, what you need is 5.0.52+ (See previous post)
To get that on debian lenny, You do not need to recompile or compile anything
Add the line below to your /etc/apt/sources.list
deb http://backports.debian.org/debian-backports lenny-backports main
apt-get -t lenny-backports install mysql-server-5.1
This should have you running with a key_buffer of any size you wish that your ram permits, and to overcome the 4GB limit on the MyISAM key buffer
Even though originally, key_buffer_size (The MyISAM index buffer) should have been limited to 4GBs on 32 and 64 bit operating systems, the limit did not apply any more for whatever reason the designer wanted to set this limit, and therefore, there is no 4GB limit on the key buffer size for MyISAM tables.
The 4GB limit stays for join_buffer_size, sort_buffer_size, and myisam_sort_buffer_size, But even those had the 4GB limit removed on 5.1.23/5.2.6
In any case, all the literature on the internet stating that there is a 4GB limit on the key_buffer_size are either outdated, or based on outdated literature.
Settings for read_buffer_size and read_rnd_buffer_size
are limited to 2GB on all platforms.