If you're concerned about data usage, a few suggestions...
Do the bulk of your downloading while on wifi. Beyond that it will probably be faster, it won't count against your AT&T data, since you're not using the AT&T connection. I believe that even on "stock" phones (like yours), that you can, in accounts and settings, set your phone up to only "sync" while on wifi, as well.
Two app suggestions, in that same regard:
3G Watchdog - Tracks your data usage and can alert you (or even shut off your data) if you're getting close to your cap. You have to input the amount of data in your plan and what date it resets each month. It's not perfect, but I've found it to be pretty accurate.
AT&T myWireless (may have a different name now) - This is AT&T's official app for all things mobile phone. You can check out how many minutes you have, how much data you have, when your next bill is due. It's one of the few AT&T specific apps I still have on my phone.
And don't feel silly for asking questions - we've all been there at some point!