Here's a very good article by LukeW on form layout:
http://www.lukew.com/resources/articles/web_forms.html

The main points summarised

• If completion time is to be maximised and data familiar to users, vertical alignment of labels and input fields is most efficient
• for this layout field labels need to be bolded to bring them to the foreground
• When data is unfamiliar to users or does not fall into easy to process groups (e.g. Address), left-justifying input fields works best
• this can increase the space the between label and input field but left-to-right reading culture prefers hard edges on the left (quick scanning of required data)
• try to avoid background colours and rules if possible (useful only when used to point out related groupings of information; otherwise they only constitute cognitive processing overhead)
• buttons should always be left-aligned with input fields
• primary action button should stand out visually through colour and bold type