CrossFireX with different cards

I'm new to multi-GPU but would love to get into it. I currently have a PowerColor Radeon HD 4890. Would it be possible to CrossFire this with a Radeon HD 4870 x2, getting three GPUs of power and a grand total of 3GB of GDDR5 graphics memory? I know my PSU can take it as it is an 800 watt gaming series.

Also, how much of a performance gain might I expect to see by doing this? I found a great condition Radeon HD 4870 X2 2GB for $100. Even if it doesn't CrossFire it'll be a beast on its own anyway I bet.

you should crossfire the 4870's x2 x2 lol be like 4 gpu's that would be crazy fast

I think there is a way to do this where i have seen boards that can crossfire 4 gpu's so i dont think it would be that hard just have to get the support i will look that up and see if there is some information on this

http://www.tomshardware.com/forum/268378-33-want-4870-4890-4860-4770-works-4870-cross

try this out i think your better off doing the 4870 deal with three of them