reccomended upgrade to run dota 2

sadly you're stuck. you could get a new gpu but that cpu will hold you back too much, a new cpu would require a new mobo which means new ram (plus windows) minimum for a new mobo ram cpu and gpu plus an OS to play dota 2 i'd say is $300-350 and thats without a case screen keyboard + mouse and hard drive or power supply.

possible if you get a cheap mobo (with a built in gpu) and a cheap cpu and a little ram it will fit in the 150 budget but i'm not sure if it will play dota 2 very good plus that still lacks all the stuff i mentioned before (but you probably already have that stuff)
 
You'll have to upgrade your RAM to DDR3 if you want to use any CPU that's worth buying.

You'll probably have to upgrade your power supply too. So like i said, upgrade everything. Or don't upgrade at all till you have enough money for a whole new build. $500 would be enough to build a new low end desktop.
 
The computer I'm sitting on right now has an old core 2 duo E7300 with a 9800 gt running on minimal details(except 100% render quality) with no issues last hitting or engaging in team fights.