I recently upgraded to the Intel 320 120GB SDD so I can give you my thoughts.
First, the performance is unbelievable coming from a mechanical drive. All that thrashing from the mechanical drive, and waiting, is gone.
Second, I went with the Intel 320 for the sake of reliability. I was willing to give up a little performance for what I believe to be a very reliable drive and company in general.
Now as for size. I have the $120GB drive and have everything that was in "My Documents" on it except for Video, Music, and Photos which I have on a mechanical drive. Honestly I wish I had gotten the 160GB model as I think I could have put everything on it except for large video files.