As is usual, you just need to figure out what it is you're going to be receiving across the airwaves. E-mail only, e-mail with attachments, regular web pages, regular web pages that are mostly text, and WAP pages (which are mostly text from websites that are specially configured for viewing on PDAs and cell phones) might be some examples. And, how much do you expect to use it?
It's not the same thing, but as an example, I use a Razr with Verizon, and pay $5/month for the Mobile Web 2.0 option. That lets me use the Verizon web browser in exchange for airtime minutes. It connects to a Verizon portal that lets me surf the news, sports, technology, business, etc. I can also add my own websites as favorites, but haven't found the need to do that yet. I also get e-mail from AOL and Google on it, but not my home ISP. BUT, I only use my Razr as a web browser when I'm sitting around without a regular computer with access to the Internet, and I'm bored. And I'm all good with that. The bottom line is that it's really all I want or need when I'm not at home or work (I've also hacked the phone so I can use it as a modem when hooked up to a laptop - but that's a whole different story...)
You just need to figure out what your bottom line is. What are you going to use your Treo for?