If you have Line out on your current card then that should do it Phil, without needing to upgrade the card.
I have a creative soundblaster 5.1 Live that has digital out as well as analogue and they start at around £30 I think but for just analogue out you should be able to get a reasonable one for around a tenner or less.
Of course you could pay a couple of hundred too but I dont think you would notice much difference audibly