Well, PCI will be phased out at some point. I don't think it will disappear completely for some years yet, but I do think you will probably only get the one PCI on lower to middle spec boards. The higher end boards will drop it altogether, and some already have started doing so.
I wouldn't bother looking at a D2X. It's a great card, but it's really aimed at home theatre use. The DX sounds equally as good, according to many who have compared the two, and is cheaper.
The DG is a great budget card, but as PapaLazaru said, it's only £20. Most people find it sounds better than even onboard audio found on the higher end boards. Not everyone does though. Some people just can't differentiate between onboard audio and a sound card.
There are people who think the DG is rubbish, but I think they are people who are used to better sound cards.
Can I ask what headphones and speakers you are using?