David, to be honest, the best choice for OS is Windows 7 x64. I can tell you from experience that x64, Premiere CS4 and 8GB of Ram is sooo much snappier and faster than XP 32 bit and 4GB of Ram. I have a 2nd PC with a quad-core and put 8GB of ram in it and tested Vista x64 + CS4 on it before I decided on whether or not to make the switch to Vista x64. I now have Vista x64 on my production PC and will never go back to 32bit and I envy those with i7 systems because they can easily use 12GB of Ram.
With Premiere, there are 2 areas that greatly affect performance while editing:
1) Media Cache location
2) Amount of Ram available
Media Cache stores the video and audio of the project files which are "Conformed" and indexed so next time you start up Premiere and open that project, the video and audio are already indexed. I know I am not explaining it properly but it does affect performance. Look at it this way: import a video file that contains audio, Premiere will index/conform both the video and audio, which means it is reading from the source and writing to the Media Cache folder. The quicker it can read and write, the better. This is the reason people always say you need 2 different drives.
Starting with Premiere Pro CS4, it can now create "Extra" processes to use more Ram(something After Effects has done for a while). This is because Premiere is not yet a 64 bit app unlike Photoshop CS4. Something you will learn is that After Effects is far more powerful than Premiere and editing Sequences in AE can open up many more options & possibilities. Besides Color Correction and Grading in Premiere, it really isn't that difficult to learn. However, AE is a beast to learn but it can provide so much artistic creativity that its mind blowing.
I highly suggest getting a membership to Lynda and check out their Creative section. This new section is about different artists and businesses in the various industries. I particularly like Troika and an artist Rick Morris. Troika is a company that designs all of Fox's broadcast material in addition to other networks. Rick Morris does a lot of work with Illustrator and After Effects and does a lot of work for country music channels, which is funny because he is as far from country as you can get. His work is absolutely amazing and check the link below to see his demo reel.
http://www.lynda.com/home/DisplayCourse.aspx?lpk2=618
Yes, the ASUS board supports Raid and has Intel's ICH10R chipset.
How long ago did you try Vista? I googled looking for your problems and it looks like M-Audio didn't release a good driver until some time in 2007. What you can do right now is download Windows 7 for free and try it out. If there are drivers for Vista, then the same driver should work in 7.
Also, when you tried Vista, did you try a fresh install of Vista? I really can't believe the horrid support from M-Audio. I found many people complaining about my FW410 and how it killed their firewire card and how it doesn't work with certain brands of firewire cards and that it must be the very first firewire device the OS sees. These problems spanned across both PC & Mac. All of the problems I've read about makes me believe that there was something very small and simple causing your problems. Because their drivers aren't the greatest, it could have been just about anything.