To "is" or not to "is"

[DesertRat]

Well after several months of searching the continent for a reasonably priced 2005 - 2006 X5 V8, we thing we have found the one....a 2005 X5 4.8is with 45k miles. But I have a few questions I'm hoping some of the members here on Xoutpost can shed some light on...

1) What exactly is the difference between 4.4i and the 4.8is motors?
(e.g. bore, stroke, computer, intake manifold, exhaust, etc...)
2) Ride quality: Is there a noticeable difference? I'm expecting the "is" to be firm compared to the "i"
3) Is the transmissions the same?
4) What about the suspension? (e.g. springs and shocks vs. air suspension)
5) Radiator, altenator, etc...

The reason for all the questions is if I have to replace items, is there going to be a problem because the "is" is well...and "is"?

Thanks in advance,

-DR