Do you need an SSD? No.
Will you gain some performance with an SSD? Depends on what you're expecting.
Whatever you install onto the SSD will respond much faster. But it the majority of the programs you make calls to (games and such) are not on the SSD, then the gains are less noticeable.
If you're just installing the OS onto the SSD and everything else onto a standard HDD, then you should be able to get away with something much smaller. There's a recent thread where folks here have full OS installs onto 20/32GB SSDs.
-Wolf sends
An SSD just means faster access to data stored on it. If you're not accessing that data very often, then, no. You don't need an SSD.