I'd go for one of Creatives offerings. There is the Xonar U7, but I've seen at least 3 people mention fairly recently, theirs developed a problem not long out of warranty.
Depends what you want to spend and what headphones you want to use.
Creative Omni (£56) is a decent option if you don't want to spend a lot and have headphones that don't require a more beefy amp.
If you're using headphones that need a more powerful amp, then the Soundblaster G5 (£130) is good. All round better product that the Omni and has additional features, which is reflected in the more than double price.
If your wallet feels a bit more adventurous, you could go for a Sennheiser GSX1000 (£200), but only if you have headphones that don't require a more powerful amp.
Surround effect wise, the GSX beats everything else hands down, but that's the only real reason to get one really. It can't drive more power hungry headphones like the G5 and it doesn't have other features the G5 has; such as portability and Bluetooth connectivity.
