You may be pleasantly shocked to know that a Seagate 7200.10 320GB with 16MB cache will only set you back $75-$80, and probably be faster than what you had.
Sata 3.0Gb/sec version
Ultra ATA 100 version
400GB jumps to $109 so not nearly as good a deal. 500GB for $120, 750GB for $230.
Hitachi has similar drives at the same pricepoint (320GB Sata 3.0Gb/sec
$75, 320GB Ultra ATA 100
$75) and so do the other major hard drive makers, but the Seagate 7200.10 drives seem to be the big favorites now.
I'm not a big fan of external hard drives. My favored backup methods are:
1. Two 320GB hard drives, not in RAID format. Just back up main drive onto second drive periodically. Fast, cheap, easy.
2. Get a DVD burner for $40 and periodically back up important data onto DVDs. The amount of data worthy of offsite backup is generally really small and can fit on a single DVD, which you could keep in a safe deposit box or at a trustworthy relative's in a small fireproof safe.