There's a ton of stuff you can get, it's kind of hard to be specific without knowing what you are getting and what you want to do with it.
Just some suggestions though.
Hardware:
-Screen protectors. Outside of dropping your pda, or getting it wet, the screen will get the most damage. Get some screen protectors so you will always have a nice screen and it adds to the resale value (no matter what pda you get, something better comes along 6 months from now that you will want.)
-A case. Lots of variety: hard, soft, leather, neoprim(sp?), with belt clips, etc. I always had a hard case (except when I had my Sony Clie NR70V) plus I also have a Grinder Gear case w/belt clip that holds my PDA, IR keyboard, WiFi card, memory cards, headphones and a few other things.
-Memory Card(s). You can never have to much memory, especially if you listen to mp3s or have videos. I currently have a Dell Axim and use a 256MB SD card for programs, photos, maps, pretty much anything I don't have to have in RAM. I also have a 256MB CF card for music and videos. The bigger the better, so you don't have to switch out cards to often.
-Keyboard. If you will do lots of typing, get a keyboard. IR keyboards are nice in that if you do ever switch to a different PDA manufactor or OS, you will still be able to use the keyboard. With connector keyboards you are locked in to the manufactor or even that one particular line.
Plus there's plenty more other gadgets you can add: WiFi card (I'm actually typing this on my Dell using a WiFi card and IR keyboard

), cameras, GPS, FM cards, Bluetooth, cords to connect to a cell phone, PowerPoint presenters, plus some I can't think of right now.
Software:
-It all depends on your needs and wants. On a daily basis I do use a third-party PIM program (scheduling, to-dos, contacts), e-book reader (btw, MS is giving out 3 free best-selling e-books each week until November), Wallet program (for personal information, bank accounts, passwords, etc), photo program, and several others including a few games. It would be easier to suggest once you know what you may be needing and wanting.
What PPC are you getting?