My friend bought an AGP Saphire 2600XT 512Mb.
When trying to play Diablo or Hellgate, part of the graphics are missing or flashing. Is that a sign that not enough power coming from PSU.
 
His PSU is a generic nobrand about 4 year old! They recommend 400W or better.
 
Don't want to make him buy a new PSU to find out it was not the real problem.
 
Thanks.

Who are "they"?  "They recommend 400W or better. "
What make and model?  "His PSU is a generic nobrand about 4 year old"

You're goign to have to find out more information on his PSU.
 
What brand is it?  How many watts is it rated for?  What is the amperage across the 12v rails?  All this information should be on a sticker on the PSU.

Also probably a good idea to mention what else your friend had in his system.

There is no sticker on my friend's PSU.
 
400W recommendation is from the manufacturer as listed on the box.
 
Are these symptoms typical of weak PSU?
Ex.: You can see only part of the main character. No head or legs. Flashing bottom display.

Symptoms of a weak PSU;
-bad 3D performance
-crashing games
-spontaneous reset or imminent shutdown of the  PC
-freezes during gameplay

 
 What drivers are you using ? there is a known issue with that card in AGP flavour.
 If you are not already using them try these.
 http://support.ati.com/ics/support [...] onID=31542
   Mactronix

Thanks for the information. I will try the driver first. I will let you know how it turns out.