Hmmmm not sure if I should buy Final Cut Studio 3 now or not
I think youve answered your own question and if your current tools arent constraining you, I dont think theres a burning need to upgrade.
Although the new features may not interest you, overall there were a number of improvements that would fall under the modest, but handy category and its really these that provide the enhancements, rather than the new bells and whistles. As with all of these type of things your own mileage will always vary, so I would take look at some reviews/forum threads to see if theres anything that you think would be of use to the way you work.
FCS 3 doesnt have any of the SL enhancements from what I remember, quite a few people suggested that this was a major reason that Apple released it before SL came out, rather than after. A while ago, there was speculation that Apple would release a 64-bit version in 2010 and offer an updater for a (fairly) nominal charge like it did for FCP 5.1 (which let the application run on Intel machines, if I remember right and if I dont Im sure some kind soul will)
but thats just speculation!
With the upgrade prices, although I was (pleasantly) surprised that Apple charged the same price for upgrading from FCP as it did from FCS 1, personally I think Apple had been pretty good with this previously and you didnt have to have the most recent previous version to get the best discount when upgrading (e.g. from what I remember the upgrade price for FC2 was the same for the Production Suite as it was from FCS 1 and most of the upgrade cost from FCP was the same no matter what the version of Pro you had).
Because of the upgrade pricing in recent years, if the 64-bit version is going to be a standard upgrade release (e.g. FCS 4) then I would feel confident that it would cost the same whether I was going from 3 or 2.