Go for the 120GB, or even something over 200GB if you can get a good price!
I have a 60GB and every single day I have to worry about things like cache or temporary files from projects I'm working on filling up my disk, or other stupid little things like that. I'll be getting a 256 GB drive soon just so that there is less to worry about. It will definitely be nicer not to have to move stuff around all the time.
@Vortex - I do not recall that X58 support SSD caching. I should deepen the definition when I say 'misc software': Drivers and utilities (hardware ID/ OC / monitoring) and programs that can "only" be installed on C: drive such as EA Origin t('_'t) OpenOffice, Guitar Pro 5, Google Chrome, Dropbox (ect.) are programs that I mostly use and will also be installed on the C: drive.
@Major - I'm usually good at keeping files away from the C: drive. Shouldn't be a problem!
Oh, forgot you had an X58 board. Caching is all run by software though isn't it? There are 3rd party softwares out there for SSD caching instead of just using Intel's stuff. I don't know if it'll actually work on an X58 board or not though.
I mean, this is just up to you. Either you'll use the extra space or you won't. If you don't think you will, then there's no point in spending more on a larger drive.
If the need arises in the far future, getting a 512GB (or whatever is standard by then) and use the 60/120GB for caching is also an option. By then I'm probably not on the X58 platform :P
For the person who made the very valid comment about temp files and such from windows, and other browsers, even some games: Most of the time, if you know where to look, or what you're doing; you can write code that tells windows to put those files on a separate HDD. The only convenience to having these files on your SSD is that the speed of the SSD is a huge benefit in speed if you want to revisit said pages.