I am a weirdo who has every card mentioned in the previous posts, and a couple more.
My card in the Main PC is the creative ZXR. I am 95% happy with it. I have found, that when I play certain tracks and have the volume on the generous side, it has a kind of "drowny" fee lto it, almos tas if when it has a strong bass line going, at teh same time the kick drum bangs out a tune, its as if the filters find it to much work and the volume drops... Ever so slightly, and most people simply cannot notice it, but I do... I have been and spent a small fortune on my audio setups in the house and I do notice it.
Last week, I swapped it for the ASUS STRIX purely cos that has been back in its box for a good year or so now, and I think thats a slightly better sound ( although in the past I was on the fence... Withotu any FX it is, but with FX the creative is better ) but after a few days, I decided not to use it as it seemed that games would kind of slow down a jot... Since going back to the Creative, the Games are still jittering so bugger knows why, cos they never did before I tried swapping them over. Oh yeah, and the headphone socket taht connects to the case isnt on the Strix but it is on the creative.
MY Main Linux machine has been using the Xonar DGX for a while and this was only used for a giggle... The XFI Fatality that I normally use was, like teh strix, on the shelf, and I have just swapped them back over, and weirdly, Linux isnt seeing the XFI and it was seeing the Xonar DGX, however I was using the Optical output and its probably me, but it had issues.
I might put the STRIX into the Linux PC to have a play.
I bought the Z906 speakers a few years ago, and I was utterly underwhelmed, however, it has multiple inputs and I found that handy because I am using a DVI KVM to go between my Win, Lin, Hackintosh and Server Machines and so I forced myself to stick with it, but I have put my mission speakers onto it cos teh supplied ones are awful... Its acceptable sound quality but in no way can be anything professional.
All these cards are great and you cannot really go wrong with them, they all have 5.1 ( or higher ) and they all have both Analog and Optical outputs ( Strix has Digital but not optical )
I wil be honest, but the DGX is a fantastic little card considering you can get them for almost nothing these days, as Reality Bites said ,£12? - go for it at that price!