"2 x PCI Express x16 slots, running at x16 (PCIEX16_1/PCIEX16_2) (Note 1)
2 x PCI Express x16 slot, running at x8 (PCIEX8_1/PCIEX8_2) (Note 2)
(Note 1) For optimum performance, if only one PCI Express graphics card is to be installed, be sure to install it in the PCIEX16_1 slot; if you are installing two PCI Express graphics cards, it is recommended that you install them in the PCIEX16_1 and PCIEX16_2 slots.
(Note 2) The PCIEX8_1 and PCIEX8_2 slots share bandwidth with the PCIEX16_1 and PCIEX16_2 slots espectively. When PCIEX8_1 is populated with an expansion card, the PCIEX16_1 slot will operate at up to x8 mode; when PCIEX8_2 is populated with an expansion card, the PCIEX16_2 slot will operate at up to x8 mode."
It should run at 16x,x8 based on the information above since PCIEX8_2 (Slot 4) shares bandwidth with PCIEX16_2 but will not run above x8. It will provide better cooling but it will make plugging in your headers, USB ports, etc. very difficult since the card will be covering most of those slots! I would in the end, recommend running the cards as they are designed and adjust just your GPU's fans speeds accordingly to help with temps.
Example, I run my two cards at 70% fan speeds versus auto (which is 30%) and my cards stay cool.
5870's are one of the (very) few pieces on the market that will actually benefit from x16 operation. The main thing for cooling is to keep from 'crowding' them - they'll intake sufficient air and blow it out of your case; what you don't want is another hot, unshrouded card next to 'em on the intake side. I'm very interested - thinking about doing some OpenCL financial workstations using three or four 5870's... Do you have the two gig, six monitor cards? I'm curious to know how they xFire, and whether the 10.5 drivers allow setting a single monitor in portrait mode yet?