I haven't bought any big disks for ages, so I'm a little out of date. I've looked at the curve of price-per-GB, but I don't have a handle on which disks people feel good about.
What size of disks would you be happy to have in your server?
Four 500GB SATA disks, might have to upgrade again later
Four 750GB SATA disks, might possibly outlast the server
Four 1TB SATA disks, will very probably outlast the server
Which disks would you be happy to have in your server?
Anything else you'd like to say?
I'll be recovering the existing disks -- two to redeploy as hot-spares in servers of a similar age, and two for desktop systems.