The following is from Handspring's web site:

How much data do I need?

Selecting a plan with PCS Vision allows you to wirelessly access email and the web on your Treo™ 300 communicator. In order to keep excess charges to a minimum, it is important to choose the right plan for your data usage patterns. We suggest the following guidelines to determine how much data you might need.* You can always call Sprint to change plans if you find that you need more or less data.

For Treo 300 users that are… …you will need… …which will let you do*…
Voice-only users 0MB per month Phone calls only
Light data users 2MB per month 375 emails or 125 web pages per month
Typical data users 8MB per month 1,500 emails or 500 web pages per month
Heavy data users 20MB per month 3,750 emails or 1,250 web pages per month

* Assumes average email size of 5.4kb and average web page size of 16.3kb. This table is for general guidance only. Your actual usage may vary. We suggest you review your first monthly bill for your actual data usage and modify your plan accordingly.