According to a Serbian Dota 2 beta tester - Alien Swarm is the best example for comparing the system requirements for Dota 2.
If you can run it (Alien Swarm), you'll be able to play DOTA2.
Dota 2 Minimum System Requirements
(Based on the Alien Swarm minimum requirements)
Windows 7, Windows Vista, Windows XP
Pentium 4 3.0GHz
Hard Drive Space:
At least 2.5 GB of free space
DirectX 9 compatible video card with 128 MB, Shader model 2.0. ATI Radeon X800, Nvidia GeForce 6600 or better
DirectX 9.0c compatible sound card
Alien Swarm is a free to play top-down shooting game available on Steam.
Download it for FREE: