when you first get it the drive will seem huge and great... but soon enough you will find a way to fill it up. you can ALWAYS use more hard drive space so if you have the money get it!
Only you can know how big your music collection is, if you intend to edit video, if you intend to download A LOT. We can't read minds here you know, how the hell are we gonna know how quick you will fill it!?
I recently moved from an 80 GB drive to a 160 GB drive. I don't do any video editing and have nearly 1600 songs in iTunes.
The 80 GB drive was almost full when it was failing and, even though I'm constantly checking the occupied space, the bigger drive is down to 93 GB free in just a couple of months.
I don't imagine that you'll need the space but we do tend to grow to whatever space we have available.