troubleshooting Radeon X300SE display problem

[flyingsolo]

I just recently upgraded from HH to 11.04 & it seemed to go well (on an old Dell box) except for display not right yet.
The initial Ubuntu splash screen & login screen look fine but once it loads, the display is not right--there is a very fine pattern of lines across screen that nearly obscures what is on the desktop or in any window; it's partly legible but not really usable.
Video card is Radeon X300SE; I've read some posts where others have it working (driver issue?) but haven't found the fix.
Thanks for assistance.
edit: here is a screenshot to illustrate what I mean
2nd edit: I admit, the screenshot doesn't look so bad when I view it on another computer (iMac) but in 'real life', it's much worse on the Dell monitor.

[flyingsolo]

So, in a way, I've fixed it by installing fglrx.
When I rebooted, there was an error box flashed for a short time that said something to the effect, "Your hardware isn't up to scratch for Unity; select Classic view for Ubuntu." and it carried on booting to my old-fashioned Ubuntu desktop which does now display properly (ie: I can see & read stuff!).
I didn't actually select classic view, it just came up with it. So, bottom line is my Dell is too old for modern Ubuntu. But then again I hear a lot of people grousing about Unity anyway and I'm Ok with what I've got.
Maybe this helps someone else trying to work with old computer guts!