zanta,
About your two evga 7800xt KO's and already purchased Creative X-FI XtremeMusic card:
I didn't realize some 7800GT and I guess GTX(?) cards were "single slot" form factor.
I'd heard some graphics board card & cooler designs were designed to take up two
slots and I'd just assumed all the 7800's were like that.
From the picture you linked:
http://hothardware.com/image_popup.cfm?image=big_slipipes.jpg&articleid=739&t=a
...it looks like the depicted graphics cards are 'single slot width cards' and you could
stick in a PCI card in the free area next to the very edge of the ASUS M/B as well as
make use of both of the PCI slots between the PCI-E SLI slots if need be.
Unquestionably if you had a SLI pair of "two slot wide" GFX boards then the only
possible PCI slot to use would be the one in between the SLI pair which is farthest
from the fan intake of the PCIE card that has its intake on inside of the SLI pair
sandwich. Even with "single slot wide" GFX cards that PCI slot would be preferable
to use just from the perspective of keeping maximum ventillation space around the
fan intakes of both graphics cards.
The drawback is of course that your sound card might be too high to fit under the
SLI bridge connector in either of those "in between the SLI pair" PCI slots.
But really if the preferred in-between PCI slot doesn't work for you I bet you could just
use the slot on the edge of the motherboard and live with there being relatively little
clearance between the intake of that video card and the back of your sound card.
I'd just monitor that graphic card's operating temperature as compared to the
operating temperature of the other graphics care with the unobstructed air flow from
an empty slot next to it and compare the temperature readings over a 1 hour SLI
3D stress test and see how they run. If the cards can suck enough air to stay reasonably
cool with the next-neighbor slot populated with your sound card then I think you'll
do fine. I don't see why they'd go to a lot of trouble to engineer some of the
7800 cards to fit into a single slot form factor if they didn't expect people to actually
use the next-over slot anyway!
I'd probably consider sticking an "ad hob" quiet 80mm fan right behind or above the
close proximity card pair just to blow some more air between the few millimeter
space that is left between the cards and really that should be quite fine as long as your
case has decent exhaust airflow in general. In that case I doubt you'd even see the
graphics card heat up any more whether the sound card was next to its intake or not.
If you have the graphics cards and sound card already just prepare an anti-static well
grounded work area, take the sound card and one of the video cards out and compare
their heights above their gold finger areas and see if the sound card's top would likely
come up to within no more than about 4mm higher than the height of the graphics
card allowing for the slack in the SLI bridge connector and if that's so then you're
certainly clear to have the sound card fit in between the SLI pair anyway.
Good luck!