I use Fedora Core 5 + Planet CCRMA to record live audio into my PC... I generally only send line-level and mic-level into my onboard nforce3 soundcard. Unfortunately its a noisy card.

What I'm looking for, ideally, will be a card with:
XLR input mic level inputs
Line level inputs
high-impedance instrument-level inputs (not necessary, but would be nice)
5.1 output (doesn't need to be spdif)

I don't currently need any MIDI capability, optical in/out, or anything too fancy. I'm not against these by any means, as they may become useful down the road. I realise my chances of getting a XLR input are slim as well.

Most importantly I want a card that is fully supported under linux with high-quality recording capability with as little noice and fuss as possible. I'm quite picky about that.

I've been looking at M-Audio Delta 1010LT, which would do everything I need, and more. Its about $200 and I was really hoping to spend a bit less than that. Also, I'd like to know if all the features of the Delta 1010LT are supported by ALSA. I've read about people who think its great and people who couldn't get it to work properly, I'd like to hear from someone who has used the various inputs for recording purposes.

I was also looking at the Roland/Edirol UA-20, I'd like to hear some about these, as they seem to have decent linux support as well.

Something like a Yamaha MW-10 would be awesome, but I do need linux support, afaik there is none for that device.

I am also picky about sound. I go for sound quality instead of looking at signal to noise ratios or THD+N. I have an Audiotrak Prodigy 7.1 that has superb playback and record sound quality. The Audiotrak Prodigy 7.1LT will be even be better because it uses tantalum capacitors which are very good for audio. These sound cards uses unbalance connections, so you need an balance XLR to unbalance converter to connect to your professional equipment.

Recording with the Audiotrak Prodigy 7.1 is a piece a cake compared to other sound cards that I have used.

Thanks for the heads up, I hadn't encountered AudioTrack before... The Prodigy 7.1 & 7.1 LT look quite nice. Linux supports all the features I assume? For the price they look to be a better value for my needs then M-Audio's stuff, especially if they achieve high sound quality.

As far as balance/unbalance, have you tried running a small mixer board into the card? I was thinking of this as a possible solution, instead of my massive collection of signal & connection adapters, just getting ~$100 mixer like the Yamaha MG10/2 for example. I've even heard good things about the Behringer UB502, which has a 1/4" unbalanced stereo output.

ALSA developers still have not provide support for 512 and 1024 oversampling which will probably provide better sound quality when playing and recording audio.

Any sound card that has line-in can accept any unbalanced audio.

You can try to ask the people at diyaudio.com about what professional mixer sounds better.