The problem with those 500+GB drives is that they're actually 2 hard drives in a striped array, so that half of the data goes to each drive. this means that, if either drive fails, you lose everything, so you're basically doubling your chances of losing your data. For backups, it's much safer to get two 250GB single-disk drives and halve the risk of losing all of it. If you have to have a single 500GB volume, my only advice would be that the store I work at gets returns on LaCie drives daily, and they almost never get returns on more expensive drives like the G-Tech G-Raid and the Glyph drives, so something they do must pay off.
Also, if you wait a little while, there may be single disk, 500GB drives out soon. I know they've gotten to 400 a few months ago, so 500GB in a single disk shouldn't be too far away.