i use a 60 GB (55.8 formated) and i have ~30GB free after installing W7-64bit and my apps. if you want to add a game or two go for the 80+ GB SSD
that won't cut it believe me. if it's just os and 1-2 games yeah maybe.
take mine for example, old 80gb intell ssd, win7 with battlefield3 and some other stuff, mind you bf3 is around 27gb already and with additional dlc (1-2 i think) down the pipe for next year. get a least a 120/128
I think you won´t have any problem installing the OS, and Skyrim on a 64GB SSD!
You could probably get by with that, but don't you think you'll want to install some other games on your drive down the road?
I'm living on a OCZ Vertex 4 with 128 GB, and I feel a little cramped for space.