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749 Dark - matt paint, no steering damper, Sachs rear shock, Showa forks, old style swingarm, only available in Biposto
749 - gloss paint, steering damper, same suspension as Dark, new style swingarm, only available in Biposto
749S - hotter motor, Showa rear shock, Showa forks with Titanium Nitride, new style swingarm, available in Biposto and Monoposto
749R - very hot motor, Ohlins suspension all round, Ohlins steering damper, radial Brembos, new style swingarm, a few carbon and magnesium bits ('04 model has full carbon fairing), only available in Monoposto
Turning the 749 into an S would need a change in forks, rear shock and engine strip. Not worth it - you may as well buy an S in the first place. There's not a huge difference between the two anyway. I bought a 749S as I wanted the uprated suspension over the base model. The engine difference is minimal so I wouldn't buy an S on that basis alone.
If you buy the 749, and you're that bothered, I'd get the fork internals changed and stick an Ohlins rear shock on it and get it set up to suit you.
[Edited on 4-11-2005 by Twinfan]