Tip : nothing is going to go as smoothly as you think it will. Be prepared to tinker a lot with the hardware/software
But it's worth it
This. It's so definitely worth it. I started out with a single 6870 300KH/s rig, now I've got 3x6870's and 2x7950's in two seperate rigs and I'm mining away quite happily. Keep at it!