I have an AMD Phenom II x4 955
I suggest you buy this cooler: ArcticFreezer 13 Pro
The stock cooler for AMD Phenom II 9xx just goes crazy loud when you play Dota 2.
If you get this cooler it's very important to place it with one exact move and fix it with the screws. If you jerk around too much the thermal paste can get to thin or air can go under it drastically reducing heat conduction.
Furthermore My 955 stock 3,2GHz can be OC-ed with the following settings:
1.3875 Volts, 203 Ref clcok, 19,5X multipiler = 3965Mhz~ 3,96GHz
Does not heat over 53-54 °C while playing Dota2 if my PC is taken out from the desk and the sides are taken off. But if you wanna OC get the ArcticFreezer Pro! DONT TRY TO OC WITH STOCK COOLER!
Oc-ing the CPU gives me an increase in fps from 65-95 to 85-120
If you want constant 120 fps with the current Dota2 go for a Corei5 2300 or better.
Also I have a GTX 560 Ti And I'm using 16x aniso + 8 AA and the card can still deal with the game, the only limitation for me is the CPU.
So a 550 Ti may perform somewhat weaker but you should be fine by not using AA.
Oh and if you use AA, use it from NVidia control panel, Dota2's inbuilt AA sucks balls, it makes the game blurry as hell.