I have an old Pentium 4 with an Audigy 2ZS Platinum in it. I gave it to my parents when I built this one I have now. It now sits beside their newer Phenom tri-core 780G system. I've tested both with my Altec Lansing 4.1 surround sound system, and the Audigy still sounds better. In stereo mode, they seem to be the same, but above that the difference is quite a bit. Especially using CMSS-3D for music files. Once you start listening to music in surround sound, or anything else, going back to stereo is hard.
The other noticeable improvement is headphone surround sound when gaming. It does make a difference in FPS games. If you ever do get a model with the bay, well the I/O options are better. Depending on your card, there is also gameport support if you still have a legacy joystick. Windows 7 still has gameport support, though Asus is going to have to bring out some newer drivers for it to work from my motherboard.
Basically, in short, if you have good speakers, especially something higher than 2.1 then your Audigy will sound superior. I did use the onboard audio on my motherboard with my 7.1 setup and it did work fine. But the one thing I did notice is some feedback problems at higher volumes, especially when gaming. Obviously, that's probably due to electrical interferance since the chip is on the motherboard. My parents system doesn't seem to suffer from that, but they are only running in stereo mode. I have come across a few games that I had to mess around with to get the sound to work properly, such as CoD4.
If you're into high-fidelity, the realtek chip does not produce true 192KHz sampling. This is done by software emulation or doubling up 96KHz, in stereo only. Your Audigy will do true 192KHz sampling and I believe up to 4 channels, or at least stereo anyways. It's been a while since I looked at those specs, way back when I built this thing, but then I read about X-Fi and waited that to come out instead.
You can test that out by playing back an Audio DVD and switching sampling rates within windows on your Realtek chip. When you select 192KHz, watch your CPU usage massively spike.
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