I'm purchasing a another Asus 5870 to crossfire on my Gigabyte EX58-UD5 mobo. My issue is that the top two PCI-E slots are 16x but the 3rd one is 8x. If I use the top two slots, the cards will be right on top of each other and the first card might burn up. Should I use the 3rd slot and have 16x on top and 8x on the bottom? Would I see any performance decrease?
My motherboard places the top 2 PCI_E x16 slots on top of each other like this. Before my 5870 I tried to CF (2) 4870 and the top card would top out at 100C+ and would crash the drivers. I ended up selling both for a 5870 because I didn't want to deal with the heat. If I had a side case fan that circulated the air between It would probably have been fine.
Well 5870s run cooler,don't they ? (I agree that they produce more heat but i think they will be fine) http://www.anandtech.com/show/2841/26
Anyway to OP,if you want,you can go for HD 5970 which performs about 10-15% slower than 2 5870s but runs cooler,quieter and has less power consumption and also its cheaper too
It seems that there are some drawbacks to mixing bandwidth between slots when crossfiring. This is more profound though with a 16x/4x configuration than a 16x/8x configuration. I do expect you will loose a little performance compared to running the cards in 16x/16x mode, but hopefully not too much. You may also want to consider getting a gigabyte card with the inverted cooler (either the Ultra Durable or the Super Overclock) and placing it as the first card so it's not totally starved for air with the other one right next to it.