I beleive that running over USB there wont be an issue on drive size. you could in theory have a 100 TB disk on a usb port (i know they dont exist yet)
You need to make sure your operating system can support a large HD tho. if you're on XP make sure you install the latest service pack and you'll be fine with an external 500GB disk, vista will cope fine too. if you are on '98 or me forget it. don't know how well 2000 deals with big disks either.
as for using it as a slave, what skruntie is suggesting is that when you have finally bought a new mother board you can take your 500GB disk out of the caddy and put it in the PC as a regular internal disk.
K7S5A you say? I had one of those. Don't buy a caddy with a GS18** controller chip as they are incompatible with VIA usb controllers. If in doubt either buy a belkin USB2 card or get a firewire caddy instead of USB2.
Trust me, I'm an IT analyst
