I'm not sure if spending loads of money on, IMO, valves on motherboards or racks of special equipment is worth it. It's all to easy to get wrapped up in all the 'goodness' of various bits of equipment, soundcards, valves and general subjectiveness. If the 'colouration' or sound you are after is worth it by using a valve compressor then yes - but is it truly noticable for any listener other than yourself? This is a debatable point.
Considering that in most cases the audio path is purely digital (unless you are specifically using analogue equipment) then nothing really matters. The audio from your sequencer is recorded to cd (or whatever) verbatim. What matters from then on is the amplifier and speaker system it is played on.
All that is required for sound output from your PC is low noise and flat frequency response. If you are using MIDI equipment then the lowest latency (in terms of audio buffer size and hardware acceleration capability) is best. Many soundcards are this good, even some pretty cheap ones.
My cheap motherboard however has HORRENOUS audio output, but my bog standard soundblaster live PCI card has a surprisingly clean output. I don't think moving the D to A conversion outside of the PC would make much difference, but there would be a tiny improvement. However the improvement would be barely perceptible - be ready to get out the spectrum analyser! The signal to noise level is too high anyway for it to be worth it.
Software synthesis and signal processing is just as good as separate hardware - after all they are one and the same thing except one is in a physical box. If you can do it just as well in software then you don't need the somewhat expensive hardware.
Heh of course it doesn't matter in the end we all have our hobbies or madness we want to spend our cash on. I built myself an amplifer that cost me more than I could buy a normal one in a shop. But hey it was something to do and pleasure is more important than money! (
http://www.psy-forum.co.uk/forum/showthread.php?t=35859)
And it is certainly pleasurable to look at a rack full of tasty equipment....
(even if you know you've only gained a few dB of S/N and improved the response around 12kHz
)