I'm no expert, I tried to Google without much luck for a comparison.
I do remember a video by a successful disc jockey who used M-Audio speakers (I think AV40's) and an M-Audio sound card.
I have M-Audio AV40 speakers. It's interesting how often a cheaper set of speakers sounds roughly the same as my speakers but how much different they are for quality audio. I absolutely LOVE my speakers. I can listen to audio without my computer by using the front input connected to my MP3 player. I can't remember if it's included but the wire is a "Male-to-Male 3.5mm Stereo Audio" cable.
All cards are PCI. PCIe support isn't a big deal. Eventually PCI will be dropped. Audio cards don't need the extra bandwidth rather it just makes sense for all motherboards to go with only PCIe to save money. It will be many years until PCI disappears completely from motherboards.
Many people who get Pro audio boards also get the front component but that's if you need access frequently but I'm no expert anyway.
ASUS has only one "pro" board and that's for a Home Theater setup not audio processing in the professional sense.
A recent article discussed gaming audio (which would include music). The conclusion was that most onboard audio solutions now provide very good audio and that the extra money was FAR better spent on good speakers. It was also surprising that many people had expensive audio cards with poor speakers which wouldn't sound any better with the onboard solution they had.
M-Audio AV30 speakers are about the best stereo speakers you can find for the price.
I think the Z-2300 were rated really high for Surround Sound but there wasn't anything that offered the BEST of both worlds.
(After my ATP5 setup broke so that the sub wouldn't work and only "Stereo" worked I put the Front-Left and Front-Right on either side of her monitor as per normal and the Rear-Right and Rear-Right on top of the monitor. I could select Stereo for just two speakers or all four and I really loved how all four spread the sound out. If you have a 4.0 or above and have space issues try this.
I love my AV40 stereo speakers but I'm considering adding a CENTER and SUBWOOFER. I'm going to contact M-Audio for advice.
One great thing about the AV40 speakers is that the 4" drivers push a lot of air for the BASS. As good as headphones get, most people fail to understand that it's not just your ears, audio "sounds" good when bass impacts your body. This is also why tracks are mixed specifically to sound good in a set of earbuds and also why earbuds usually suck for listening to movies which are NOT mixed this way.)