'Güzel' means 'Beautiful' so 'Güzel bir kız' means 'A beautiful girl' But 'Güzelce' means 'Beautifully', it doesn't make so much sense in English but 'Güzelce bir kız' literally can be translated as 'A beautifully girl' actually both has same meaning. You can use the one which sounds good to you. But I can recommend you to use 'Güzel bir kız' better.