SLI Q's & gtx570 enquiry

Hi,
I looked around and wanted to clear up how sli actually works, like when i'm running a single lane 16x or two 8x lanes, what benefits does sli actually give, more shared memory? or parallel processing, core speeds etc. does the 8x not really matter (in my head i just keep seeing it like a 3ghz single core vs 1.5ghz dual), and with multiple screens do all plugs just go through one card or does it not matter where you put the plugs

also with prices dropping and people selling 2nd hand cards cheap, i bought an asus gtx570, it takes up 3 of my 6 slots, any problems or issues with installing two almost touching each other,(one basically sitting on psu and other would be on top of that) i get the feeling it's not ideal but also have been told that the asus versions run very cool with the larger heatsinks

thanks if anyone can answer some or all of my questions