Power is going to be the main issue. My old and trusted Giga-EZ 1000W should be enough for GTX 480 in SLI, but to accomodate 3 of them?
I'd go with a Antec 1200W unit cz what I need a SERIOUS power supply for that...
I'm more interested in is triplehead benches though and the scaling on 3-Way SLI is better with a highly overclocked Core i7,4800MHz for a start
Don't listen to them, they are going off of mis-information.
Your OC will be limited, but Tri-SLI GTX 470s or 480s is perfectly doable when it comes to heat.
You will get away with a GOOD 1KW PSU with 3 GTX 470s, but you will need a little higher if you plan to use 3 GTX 480s and overclock them.
I recommend 3 GTX 470s instead of 3 GTX 480s because I have seen people get them to ~800MHz core in tri-SLI with 80-100% fans so it is perfectly doable. The GTX 470 is only about 6-7% slower clock for clock than the GTX 480 unless you are playing at resolutions over 1920x1200 with heavy AA.
Either way you will need a new PSU, because your 1KW PSU is of questionable quality.
Yes in part because the cards can never be driven to their 'theoretical maximum' its not going to be linear with the FPS scaling but 'sorta' similar. This was proven with Hocp's sound and watt test. GTX SLI drew less than 600 in sli , or less tha 200 for the second card.
Well if you have liquid, that changes everything. You will need at least a high end pump and a triple and dual radiator, or a quad radiator to cool those. A high end pump with a good head will draw 100w pretty easily.
As for dual PSUs, sure its pretty common actually. All you need is an adapter to attach to the secondary PSU's 24pin motherboard connector so that it will trip on with the other PSU and think it is plugged into a motherboard, you can solder some wires too but this is easier without destroying any resale value of the PSU.
Some cases that support dual PSUs, to my knowledge:
CM HAF 932
CM Stacker 810
CM ATC 840
Silverstone TJ07 (my case)
Lian Li PC-A70(71)
Lian Li PC-A7010(7110)
Lian Li PC-A77
Lian Li PC-A70F(71F)
Lian Li PC-P80