According to this thread on AMD Developer Central, this card is too old to be supported by the proprietary fglrx driver.
Also, it's not suggested to download the driver provided on AMD website.
Therefore I think the only working driver is the open-source one.
If you're having graphical performance issue, you could:
- Try older LTS version (e.g. 12.04, but I doubt it will help in this case.)
- Select a light-weighted flavor Ubuntu like Xubuntu or Lubuntu, which needs less graphical power.
More information from RadeonDriver on help.ubuntu.com:
Supported, but Hardware is Too Old for Unity
These cards will not run Ubuntu's Unity desktop with 3D acceleration. They will still run Unity, but the CPU will be used for basic drawing and performance may suffer. If you have one of these cards, a lighter desktop (such as xfce or lxde, found in Xubuntu and Lubuntu respectively) is recommended.
R100 Radeon 7200
RV100 Radeon 7000(VE), M6, RN50/ES1000 (2D only)
RS100 Radeon IGP320(M)