There are many apps on the Market that can track your data. If you search the Market for "Data Useage" or "Track Data" you will get quite a few results. However reading the reviews for each seems they are not that good. If I had to choose I would use "My Data Manager". They have a free version with ads and a paid version without ads.
It is easy to setup. Set what data you want to monitor (mobile, romaing, wifi). Mobile in your case. Set your limit and your billing date and you are set up. Gives your total use and what apps are using the data.
And I just happen to have a copy of the paid version for you to try.
Install on internal memory and do not move it to the SD card. Give it a try and let the forum know what you think.
Also, not having to do with this app. At anytime you can call this number *3281# and ATT will send you a free text of your monthly data useage. Useful in checking the app for reliability.