I’d recommend something a little more lean than Ubuntu for older machines. I played with several distros several years ago, and I found Ubuntu as much of a resource hog as MS Windows.
If it’s a really old machine, you might try Puppy.
http://en.wikipedia.org/wiki/Puppy_Linux
Last night, I needed a fresh Linux virtual machine. So, I decided to give Ubuntu 14.04 Final Beta a whirl. I was using Virtual Box, hosted on a seven-year-old Windows box in the basement and accessed from my laptop via Remote Desktop. Definitely not a high-performance graphics environment!
It sucked. They've added some fancy visual window transitions to Unity, and they were making the system crawl. Running top in a terminal window (after patiently waiting for it to open in response to Ctrl-Alt-T) showed compiz taking a lot of time. Allegedly, there are tools to turn off the graphics hacks, but they didn't do anything. I'll have to try it again in while, using the release version when it becomes available.
So, since fancy graphics wasn't really my focus, I deleted the new VM and made another one using the Ubuntu 12.04 Desktop ISO. It runs fine.
I also have 12.04 on a $300 low-powered real machine (Intel D-525, 2mb RAM, 40g SSD). I used the Alternate Install CD and set up a Flex-based window manager with a highly customized menu. It is very snappy and plays videos nicely. I suspect it would also run 12.04 Desktop just fine, but that wasn't my need.