Triple SLI and Triple Crossfire seem to work far better than quad anything from what I've read about it. But, it's been a few months since I researched it at all. Things may have changed...
No, you cannot mix ATI-based cards with nVIDIA-based cards. There are some cases where ATI owners have been able to install nVIDIA cards to function purely as GPU PhysX cards, but this works only in XP and Win7 beta, presently. Vista does not allow for the drivers for each to be installed.
The BEST combo so far, will be Tri-fire GTX285, but take a look at tri-firing a 4870X2 with a 48701GB, it is rather cheap (for what youre getting) and also, only takes up two PCIe slots, which expands your motherboard choices alot.
I have a tri-fire setup, and it has done an admiral job for me.
Yes, triple-SLI means three GTX285 cards. Yes, you can CrossFire a 4870X2 with a 4890. Most will say that when you don't match cards, the higher clocked cards will be downclocked to the level of the slowest in the pair/bunch. This is not always true. There have been some cases reported where the cards in mixed configurations retain all their original settings. Pretty much hit or miss with that one, as no one's debugged it.
Anything with a bottom mount PSU is advised. I have a CoolerMaster RC-690 and it works very well for my needs. Many people will recommend the Antec 900, or the newer version, the Antec 900-Two. Both are excellent, and with three cards, you should consider them due to their increased front air intake and large side vent (which can also support a fan).
PSU is on the bottom, as you said, but i dont know if it could fit my asus delux along with 3 cards and still have a good airflow. What do you think? 900 is good and all, but this one really steals it for me. I really like the design of the performance 182.
They are attractive cases, but you will probably want something with side vents or side-fan(s) for a Tri-SLI setup. On a sectioned case like the 182, the area where the lower cards would be installed would become a bit of dead zone when it comes to airflow.
I use a coolermaster HAF and it keeps the airflow at all key areas very strong. I would say you may not "need" a side fan, but you will need some supporting in take for the lower portions of the case to give the cards some fresh air to breathe or they will be getting toasty for sure.
If you are spending this much cash on a video card setup certainly don't skimp on the case. That antec is a very snappy looking case.. my brother has one except he painted it and cut it up a bit for cable management.. however it doesnt provide all that much air flow compared to others. It is a very nice case for water cooling though, roomy and whatnot.
For air cooling I woudl recomend something like the Coolermaster HAF, Cosmos S, Antec P193. The antec p193 is pretty sharp looking.
If money really isnt that much of an issue, and style is really importnat, have a look at LIAN-LI, such as the PC-X2000B or a custom PC-P80
The haf has front 230, side 230, and top 230.
Cooler master cases are built really well and have great customer support. You could not go wrong with wither the haf 932, cosmos, cosmos s, atcs 840. All have great cooling, plenty of room, and great build.
The Cosmos S case is very attractive, and can be loaded with fans. ( I have 7 X 120mm fans in mine) Plus, you can use the giant 200mm fan for video card cooling, althought I took mine off and replaced that side of the case with a window.