Att wireless (wifi charges!) htc 8125?

hope smeone can answer this- i have att wireless i had a nokia 6126 flip phone was not working well. i bought a cingular 8125 pda off evay. this has wifi on it. now like and idiot i called att to let them knw and they changed my ie number so they know i have an 8125. now i had a text/data package on my phone before but they always want you to get the unlimited data for like 40 buks. cant afford that i asked if i can still use current plan and i cna tell they are trying to push me to get the 40 buks a months cause i have a pda. also they tell me that if i go on to wifi somewhere i will get charged. i thouht it was free. i heard if you take out your sim card then go on they can ttrck it. is this true. aaahhhggg!!!