The CPU doesn't meet the minimum requirements for Ubuntu 11.04:
Ubuntu Desktop Edition
1 GHz x86 processor (Pentium 4 or better)
512 MiB of system memory (RAM)
5 GB of hard-drive space
Graphics card and monitor capable of 800x600
Either a CD/DVD drive or a USB port (or both)
Internet access is helpful
I recommend installing a "lightweight" desktkop environment (like LXDE or Xfce) instead of the standard Unity.
You might also consider a distro designed specifically for older hardware, like Puppy or SliTaz.