I just did this on my 9300 over the weekend, after reading the Maximum PC article on the subject. I'm a complete noob at this, but here's what I know:
You have to use a separate partition on your hard drive to install ununtu, as it uses a separate, incompatible file system. I used the 4Gb hidden Dell service partition just to try it out - it seems like a more fitting use for the space to me (I already burned my backup XP cd using dell's utility.)
Anytime you have a question about updating or installing something, check http://wiki.ubuntu.com
first - the best solutions are usually the ones I find there.
Use the synaptics package manager in ubuntu to install the nvidia drivers (and most other stuff you need), works like a charm.
When you're done installing ubuntu on your drive partition you'll see a list of OS's to choose from when you boot up. Mine defaults to ubuntu so I have to watch it and select XP if I want to use windows; otherwise it times out and boots in linux.
My next challenge will be updating to the 686 kernel (ubuntu installs a 386 kernel by default.) Again, I think this will be easiest with the synaptics package manager.
Have fun! (and back up first!)