I think platinum is the 'in' thing, if there is sort of thing in wedding rings. Look at what she normally wears. If she wears more silver than gold, go with either platinum or white gold. If she wears more gold (yellow gold, that is) then look in that direction.
If it doesn't need to be a surprise, then just ask her.
Most women have a pretty good idea of what they'd like in an engagement ring/wedding band. In fact, there's the whole issue of solitaire versus more of a 'fashion' ring that will eventually nestle in with the wedding 'band'.