Those are excellent bargains at the moment, about 3 months ago they were near 200 dollars. 130$ now, same as the 7770.
If you cant buy from newegg and you're looking at a place that will ship to you, make sure you look for the EVGA superclocked 560. If you try and get a 560ti the price will jump up, it's a different card.
crossfire is a nice idea but not as smooth as a single card. better to have a $200 card than 2 $100 cards. also saves power. and if your motherboard supports virtu MVP you will definately want one card. it makes games so responsive and smooth.
from what ive found dota 2 will run fine on any current generation card.
DOTA 2 Recommended System Requirements
OS: Windows® 7 / Vista / Vista64 / XP
CPU: Intel Core 2 Duo E6600 2.4GHz/ AMD Athlon 64 X2 Dual Core 5600+
RAM: 1 GB for XP / 2GB for Vista
GFX: NVidia 7600 / ATI X1600 or better
GFX Notes: DirectX 9 compatible video card with Shader model 3.0