Larger capacities of the same model are generally faster, but any size is going to be vastly better than a traditional HD. Personally I wouldn't go less than 240GB for a system drive. With video editing I'd think you would end up with cache and temp files on the OS drive. (Though you could maybe set it to use the media drive, but that partially defeats the purpose of having an SSD.) If you do iPhone backups to the system drive that will eat up space. If you are downloading movies you'll have to constantly monitor and empty your downloads folder.
It generally isn't much more to make that step up - the Sandisk I linked is $44 for the 120GB vs $64 for the 240GB.
Alright thanks for the advice, I went with a 240gig sandisk from best buy here locally. Now I get to install it.
They didn't have any adapters I need and I want to get this thing running!
Is there a common method for installing it without an adapter?
When you say "float", do you mean just plug it into the slot and let it hang there, or do I need to fabricate/rig anything?