Well some actual information is always useful otherwise you are getting misinformed advice. Help us then we can help you.
What was GTA performace like before the card, what exactly is wrong now (i know you say there is stutter but when does this happen or is it constant.), also post full system specs and graphical settings for starters.
As a quick guess you probably just need to update your drivers from the ATI website, if you just plonked the card in and used to packaged CD.