The problem might be your video card was released when dirt was new.
That's to say: it's flipping old. Blizz link above lists your card on the low perf chart.
Your fps issues aren't Blizzard's fault; they're yours. You can get a great video card for around 120$ that will run the game perfectly. You already spent 60, what's another 120?
Else buy a new computer without that !@#$ty "locked GPU/CPU" etc.
Find these lines:
Make sure the values both have ones.I was having trouble with perforamance until I changed these settings,and now D3 runs great on my 2008 laptop and it only chugs a bit during huge crazy battles,which thankfully isn't too often.