Audigy 4 OEM - £35. Might find an Audigy 2 ZS used on ebay for very little. Either of those will do just fine.
Definitely worth having, especially since you'll be using the anlogue outputs with the Gigaworks. Onboard is the cheapest of the cheap component wise, it probably adds less than £10 to the price of a motherboard. If you've got speakers with digital inputs and your onboard has digital out, it's not such an issue and onboard can ok, but the Gigaworks are analogue only.