I have the 80Gb and with wow and os and SC2 I very nearly ran into an issue patching to 4.0.1, it was creating temp files as it ran and my 20Gb spare dropped to under 1GB, I was just about to ninja delete SC2 when it rocketed back up.
A few friends had the install fail due to lack of disk space and had to delete stuff and patch again.
As said for logging into very busy places an SSD and WOW = awesome, normal zoning, raiding etc you will see limited diff compared to a fast HDD.