I have an old laptop that I would like to install an old version of gnome on. I am new to debian and Ive been running sarge for a couple of days. But what I want to know is if its possible to choose a different version of a package to install.

I run gentoo on my main box so for example to install an old version i would type Is there any way to choose different package versions with apt-get or can i only use the default?

yeah , its the same

apt-get install k3b=0.11.12

Well it's similar. You need to make sure you have the proper sources in your source.list file located at /etc/apt/source.list. The entries will say something like
" deb http://webaddress.whatever stable main contrib non-free "
" deb http://webaddress.whatever testing main contrib non-free "
" deb http://webaddress.whatever unstable main contrib non-free "
If you want something from testing then you would add the line that will use those repositories.
The difference being woody. For unstable you would use unstable or sid. Don't remove the other entries. Once you have those sources you can apt-get update && apt-get -s install kde/stable for example and you would get what you need from woody for installing kde and all that it depends on. You should be careful when doing that and use the -s switch to simulate it first to make sure it doesn't remove anything you actually want. When you mess with mixing the distro's like that it can get hairy. I would suggest browsing apt-get.org and check to see if there is a port to sarge that would work for you. If you are just looking for a lighter desktop and like the gnome look and feel check out xfce4.. it's fast and it's what I use on my laptop. You can also check out pinning in apt.conf but that's another post .