I have 3 PBs; so I went through what you are going through now. Here are things you may want to consider:
For viewing,
ACD 20" or/and 23"
For desktop ergonomics,
External keyboard (wired one or BT, Apple's keyboards are sweet; Kensington's slim keyboard is also nice)
Cool pad (to keep you pb cool)
iCurve (to increase your desktop real estate while keep the pb cool)
External mice (there are so many; I have the collection of mice from Logitech, Kensington, and Targus)
USB hubs and Firewire hubs (these come in handy if you are prone to plug in and out something often as the hub will get the wear and tear beating instead of the PB's USB and Firewire ports.
For additional storage or back up needs,
External hard drive (I have Lacie 250 GB, 160 GB, Arcom 160 GB, Lacie's new mobile 100 GB hard drive)
For travelling,
Various types of carrying bags (I have bunch of bags from Blooq, Brenthaven, inCase)
Extra AC adaptor (I keep the extra one in my travel bag so that I don't have to take the time to pick up the AC adapor I used at my desk)
Universal Car/Air adaptor (I have the one from Kensington)
For cleaning,
iKlear (I have not yet used them even though I have two brand new packages here; my PBs are so clean)
Micro Fiber towel to clean and wipe your PB (rough cloth will scratch the exterior and screen)
Of course, there are more to complement your PBs, such as printer, scanner, iPod, small flash drive, surge protector/battery back up, external computer speakers,................