To help you with your own independent research.
Dota 2 Recommended requirements:
CPU: Intel core 2 duo 2.4GHz
CPU Speed: Info
RAM: 1 GB for XP / 2 GB for Vista
OS: Windows 7 / Vista / Vista64 / XP
Video Card: DirectX 9 compatible video card with Shader model 3.0. NVidia 7600, ATI X1600 or better
Sound Card: Yes
Free Disk Space: 2.5 GB
Tom's GPU Hierarchy Chart:http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html
The 6670 is 10 tiers higher than the recommended. I'm sure it will do fine.