It appears that several of us here on this forum that are using Cingular have signed up for their MediaNet Plan which is 20.00 a month. They also offer PDA connect (which is what you are supposed to use with a Treo) for $44 a month. Both of these plans are all you can eat data. I also know that Cingular offers 5 and 10 mb plans per month for quite a bit cheaper. . . I am guessing for a little email, and 8 times on the net in 5 years, this would be a reasonable plan for you. The problem is, Cingular won't see the 700W or 700P for many months.
I am not sure what Verizon has in the way of smaller data needs. It sounds like you are dissatisfied with your sprint coverage, and they seem to offer the cheapest unlimited data plan of anyone . . . .
One other option, that I don't know anything about,
is using the Carrier provided email service. Its a per month charge, and might be the solution your looking for. Cingular's is called express mail.
After shopping for a PDA for 4 months, I bought my Treo from a local Cingular store the day after Christmas for a what I thought was a great deal ($250 after $100 rebate). I have been with Cingular ever since they were Bell South, and all and all have been very satisfied. Their Customer service leaves a bit to be desired, but local as well as national coverage and rollover minutes have kept me with them! I am already longing for EVDO, but if your not using that much data, Cingular and the 650 might be an option . . . . . . . .