http://en.wikipedia.org/wiki/ATI_CrossFire
"CrossFire can be implemented with varying-GPU cards of the same
generation
(this is in contrast to Nvidia's SLI, which generally only works if all
cards
have the same GPU). This allows buyers who have varying budgets over
time to
purchase different cards and still get the benefits of increased
performance.
With the latest generation cards, they will only crossfire with other
cards
in their sub series. For example, GPU in the same series can be
crossfired
with each other. So a 5800 series GPU (e.g. a 5830) can run together
with
another 5800 series GPU (e.g. 5870). However GPU's not in the same
hundred
series cannot be crossfired successfully. (e.g. a 5770 cannot run with
a 5870)"
You might want to do a little research though, on "microstutter", to see
if doing things like that has any side effects or not. I've never run
SLI or Crossfire here, and have no first hand experience with it.
http://forums.extremeoverclocking.com/t299445.html
Paul