1) Limewire is shit, use BitTorrent.
2) Unless your definition of "lots" differs from mine, 500GB is nowhere near enough space and iMacs don't have room for the addition of more drives, so you'll want to consider getting either a NAS or external HD. If you're feeling spendy the best of the bunch is the Synology DS508. Yes it's $999, but it's also a RAID NAS with 5 hot swappable SATA ports and a lot of sexy LED ports on the front.
(Even with the upgraded 1TB drive, I still think the machine's onboard storage is lacking both in space and redundancy but then again I'm a bit of a power user)
3) If you do want a Mac, and not a PC, then I think that you've made the best choice available to the average user, who would only be wasting money getting a Mac Pro. So my overall response is to get the machine and make sure you're not setting it up opposite a window or you'll never see anything on the screen.