I have a 320 GIG external IOMEGA hard-drive but, I wish that I had bought at least a 500 GIG drive since I save my images in RAW and then save the post processed images in Photoshop and then sometimes in JPEG (for sending in the web).
If you have a FRYS Electronics near you, I suggest that you visit them.
If you don't have a FRYS, here is a link to their hard drive selection.
FRYS.com
This drive runs $199 plus shipping at FRYS
Seagate FreeAgent Pro Data Mover 750GB eSATA, FIREWIRE 400 & USB 2.0 External Hard Drive - ST307504FPA1E3-RK.
It seems to have some pretty good reviews.
Seagate FreeAgent Pro Data Mover 750GB eSATA, FIREWIRE 400 & USB 2.0 External HD @ Outpost.com - $198 FS
It appears that this unit is also available at COSTCO
Costco - Seagate 750GB FreeAgentā„¢ Pro External Hard Drive
UPDATED ON 11/2/07 AT 5:50 pm PST
I just purchased a Seagate 500 GIG Freeagent External Hard Drive from FRYS Electronics in San Marcos, CA - this drive is not firewire compatible but the price was $119.00 on sale from $179.00 which was the listed price onthe box.