ASIO isn't my strong point, unfortunately. What I do know is that some external multi channel sound cards can be a pain in the ass. I have no idea what's happening inside of them, I only know that internal cards are easier. If you want to do it the easy way, get yourself a couple of Audigy or SB Live! 5.1 and use the kX Drivers. Or use different cards (I have one computer with one SB Live 5.1!, one audigy, one cheap ass USB sound dongle (only one channel, mind you), one C-Media based card and one VIA Envy based card. They play very nice together.
But I would guess that you're more of a hifi freak than me (I use VBR ripped MP3, I assume you use APE or similar lossless format, right? If not I don't think ASIO playback matters much, and you will need a very high quality system to hear any difference - my $ 10 000 home theater system (Onkyo/Mirage) isn't able to discern them, but I do hear the difference when I get anything below 192 kbps), and then those cards doesn't have high enough sound quality. In that case I would rather go for something like an internal M-Audio.