Ok, maybe its not as exciting as all that, but there are some improvements to this new version of the 2.4 kernel. You can look at the list of changes here and you can download the new kernel from the main page, here.
Generally speaking, I find it better to FTP to ftp.kernel.org and download the source from there. The path of the file you are looking for is ftp://ftp.kernel.org/pub/linux/kernel/v2.4/linux-2.4.20.tar.gz
Both the web site and the ftp server are a little slow right now with armies of Linux nerds all over the globe trying to get the new sources as soon as they can. Peace, brothers!
Once you have downloaded the new kernel sources, there's excellent instructions for building your new kernel here and especially here.
If you have any more questions, try posting about them in our "Alternative OS" forum here.
Gentlemen, start your compilers!
More at Kernel.org