What is it?
ATI Smart - ati2sgag.exe

What does it do?
This only runs occasionally: When you first install the card and after a driver update to check for compatibility and stability issues. It will determine and set the best AGP bus settings for you. Things like fast writes are set by this.

This is a service that is set to automatic. I will go against what alot of people are saying and tell you to just leave this sucker alone. It does NOT remain running after its done so leaving this alone really won't hurt anything.

Virus Precaution:
The original file from ATI gets placed in the C:\WINDOWS\System32\ directory. If you find it anywhere else then you should be suspicious for sure.

You'll want to keep an eye on this google search for any known viruses. At this time I have not found ANY viruses that run themselves using this filename. All of the results currently affect this file in some way, but do not actually run as this filename.