What do you use the computer for, and do you have any other hard drives in the system?
I left mine as one big partition, primarily because I installed the OS and all of my normal "office" applications on it. I'm still at 114 of 149 GB (formatted) free, but I have at least 4-5 more applications that need to be installed, and I wanted plenty of growing room. In hind sight, an 80GB drive should be plenty, but hey, at least I can use this one in my notebook when I'm ready. 🙂
With that said, I also have two spare hard drives in the computer for storage, media, and games. If I didn't have the spare drives, I would partition the X-25M into something like 60GB for OS + applications, and the rest for games / storage. Really though, if you can afford an SSD, you should be able to afford a secondary drive for storage.