It really depends on what kind of clients you're aiming at. You have to recognize what your client wants and respond accordingly. Although I do agree with PrintDriver here, having both is the best idea...
I've styled a form using a lot of css, I didn't apply any css to the buttons themselves but it looks like because I am surrounding the form elements in css it's stripping any styles windows XP might apply to it. I think it's called luna or whatever.
The drop down in the form is staying XP styled since it can't be styled, but the submit button is old school looking.
Is there a way to say hey button, don't change, use XP style?