I guess it depends what you use it for. I have 40GB like the one for sale, it is plenty. And I keep a LOT of photos on it. Not even half full yet. But I don't play online games or anything like that, I know they need a lot of space for that stuff.
I did a lot of learning when I was looking to buy this one. Here is some of what I learned.
The 40 GB is the size of the hard drive. This is storage. Now you can get one with 1000GB if you want to pay for it.
The 512 MB is RAM - Random access memory - determines how many programs you can have going at once.
The 1.8 GHz is the speed. Again, this is slow compared to top of the line today, but plenty good enough for my needs.