500 GB Hard Drive

I am going to buy a 500 GB hard drive soon to go with a new iMac 20" 2.4GHZ to use to backup my computer with Time Machine. The guy at the MAC store said the backup hard drive should be at least 20% bigger than the hard drive of the computer.

Anyway, the 3 hard drives I am looking at have different prices. The cheapest ($120) has a USB 2.0 connection. For a little more I can get one that has a USB 2.0 and a Firewire 400 ($160). And the most expensive has a Firewire 800 ($195). Is there any real advantage of one over another? Does Apple have a recommendation?

The only advantages are the transfer speeds
USB 2.0 - 480 mb/s
FW 400 - 400 mb/s
FW 800 - 800mb/s

I'm sure Apple recommends some drives but only for commercial reasons.
I have a 320GB USB 2.0 works fine for me speed-wise it's what i use to backup all my things.

While USB 2 is faster than Firewire 400, that speed is only achieved in bursts not sustained. Firewire 400 peaks at and sustains 400Mbs. This is significantly better for larger file transfers. FW 800 will do the same thing with corresponding results. I would go for, and use, the FW 400 if I had to but would choose the 800 if I could afford to.

If you're just using it for storage, the speed doesn't really matter. After the initial dump of all your data, Time Machine just makes incremental changes when you change your data which doesn't take any time at all even with a USB connection.

For your need, I would just go for the USB connection one and save a little money. I have 2 USB drives I use for Time Machine and movies. I watch movies all the time that are read form the external drive and they play fine, so it's plenty fast for me.

If you are getting a HD with firewire connection, make sure you check the Firewire Bus speed of your computer so you can buy a compatible HD firewire speed connection.