SSDs are getting very popular around here, especially with people building new systems and those upgrading who have their windows installation disks and no how to find/download/install drivers etc.
The current situation over here (UK) is that a 60gb drive is about 55% the price of a 120gb one and that's making people think twice about whether it's worth maxing out the budget. Either way, most users will need to use a standard HDD to store most big files/folders.
My opinion is to go with the 120gb option and here's my reason;
Look at most people's computers and you'll find that if they are music listeners they will almost certainly have between 60 and 100 (at a guess) gb of OS, software, documents and music. People going over 120gb seem to have video and or pictures collected over the years, probably passed from one PC/laptop to the next.
If I was building a new I would put everything on the SSD, for sure, except video. 120gb goes a long way without video.
BUT, you are talking about just the OS, in which case my argument is probably wasted. However, why would you NOT want to install your games on the SSD? Windows will use *somewhere* around 15gb i.i.r.c. You could then fit 6-10 big apps/games and still have room for more stuff. It would be disappointing to run out at some point in the future. Load times will be quicker on the games, although admittedly once you are up and running, you won't know.
If you start to focus on only the OS, you may as well buy an intel 40gb.....that's enough and they are reputed to be worth the extra money.