I'm currently running a 5870 with dual screens, I gather I could add a 5770 (assuming my MB/PSU supports it) and hook two more screens up to this system without messing around with eyefinity and end up with 4x screens? And using ISBoxer to tell each wow what GPU, Any downsides to this? slower swapping of wow instances between the GPU's?

and to extend it further, I could add a second 5770 if I get a MB that supports 3 video cards..
1x5870
2x5770
and hook up 6 screens to one?