I believe your system can support Dota 2 although I do note that you experience lag even at low settings.
QUOTE(AxelTG @ Jul 12 2014, 01:36 PM)
I'm not sifu or anything, but I can say your desktop is still ok for DotA 2 (the proc. at least).
Yes, you need to upgrade your GC card. But I'm a bit concern on the power supply if you are to upgrade your GC card.
If I'm not mistaken, DotA 2 is not really a GPU intensive game.
A HD7750 should do the trick (I'm assuming you're gaming on a 1600x900 monitor).
I agree with AxelTG on your PSU. Few people consider it as one of the most, if not the most important piece of hardware in your tower. Like the heart in your body, if it is weak, it can stall your whole system, worse still cause permanent damage.
Look into getting a basic 500W PSU because if you invest now it'll last a long time.
QUOTE(reiben05 @ Jul 13 2014, 04:35 PM)
I'm running a GTX 580.. maxed DOTA @ 1080p with everything enabled

.. steady 120 frames most of the time..

it should be equivalent to a GTX750/760 depending on games..
On graphics cards, I'm not sure whether you would need to go all the way into a GTX750 like Reiben suggested but that's definitely a good card to look at if you have the pocket for it.