520w might be insufficient for a 7870 CrossFire setup. Other members care to chime in?
Also, the lowest clock speed of the two cards will be taken, so if you get the PowerColor 7870, they will both run at 1100 MHz.
If I were you, I'd upgrade the current video card.
Didn't even think about my PSU when I posted this. And I will add that I just built this system back in February, seems kinda silly upgrading the GPU so soon. That's why I was going with the crossfire option. Would I really see much of an improvement if I were to add a second 7870 anyway? (assuming that I upgraded the PSU as well)