Because I have compatibility problems with Win7 I am looking for a new card to replace my good ol' Terratec 6fire LT. Which W7 compatible sound cards do you recommend for HQ music playback with at least 5.1 analogue outs and SPDIF (coax would be a plus)? Is USB good enough or should one prefer PCIe?

A Dolby/DTS decoder on the card would be a plus for movies and surround audio but it depends on the price surplus.
EAX support would be a plus because I am a casual gamer.

How does my card compare in sound quality to today's products (e.g. a X-Fi Xtreme Audio)?

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I don't think there are many USB devices that have 5.1 out.

Personally, I would probably just use the sound output built into my motherboard as many current sound chips measure quite well and electric noise will not be an issue over spdif. If you are using analog outs noise *may* be an issue.

If you really want to buy a dedicated sound card I recommend something from HT Omega or an Asus Xonar. I don't know much about the Xonar, but HT Omega makes decent drivers.

If EAX and best SQ is important, I can recommend Auzentech's X-Fi Forte 7.1. As the name implies, it uses Creative's X-Fi chip, so fully supports EAX. Also Auzentech's driver is also reported to be much more stable than the original drivers by Creative.
I don't know how it compares to your old sound card, but the Forte is a great plus sq-wise compared to my old Audigy2.

I don't know details but xbitlabs appears to have knowledge and write profound articles. They gave the Xonar as well as the X-Fi the precedence over both Auzentech Forte and Prelude in terms of sound quality?!

Or should I consider sticking to the onboard SPDIF and go out digitally to a surround receiver? My PC stores all my music. I also have many movies on the hard drive. At the moment I don't have a receiver but I am planning to buy new speakers soon and then I must decide between buying a decent sound card and linking it directly via analogue out to the speakers or connecting a cheap sound card the digital way to a surround amp. If I take the first solution I'd need a sound card or logic to decode HDMI on the PC - does there exist any? Can this be done via software? For the second solution I'd need a surround receiver with various sound format decoding possibilities and the ability to pass through the audio signal from my pc. I need a basic advice.

If you're buying new speakers, buy at least bookshelf ones, and a receiver. Satellites are way too small. And unless they have integrated amps, you won't be able to connect them directly to the sound card outputs. If you're going the receiver way, you probably won't need a separate sound card. SPDIF can handle all stereo music, probably without even resampling through WASAPI exclusive mode (if you have Vista or Win 7). If you wanna go HDMI, there are cheap solutions like the whole range of ATI 4000 cards, and the 5000 cards, but the cheap 5000 cards are yet to come out. It'd be worth it to wait until they do though, they are significantly better than their 4000 counterparts.

I want to go with bookshelves because of their size, just what I prefer

I have a Radeon 3850, with a DVI to HDMI adapter I should be fine?! Don't know if audio over HDMI works flawlessly. I remember seeing many people reporting problems.

I am not sure of what price class to look at for a receiver. I suppose the new audio formats are overkill (Dolby TrueHD and DTS Master) so I wouldn't need an expensive one with decoding possibilities for those. I'd be fine with a cheaper one supporting DD/DTS and Blu Ray decoding. But 2 HDMI ins are maybe too few, or I could connect my sources over SPDIF too. But maybe problems arise with the receiver not being able to pass through signals to output devices? I will have PC, SAT receiver and maybe a DVD player (and in the future a console?) as sources. Output will be speakers and flat TV.