I used to like ASUS a lot but for my latest computer build I used eVGA just because it had what I was looking for. The new i7 Asus boards are supposed to be pretty good. I usually check reviews on HardOCP and AnandTech before I make a major electronics purchase. They usually have good reviews and comparisons you can use for reference.

As far as overclocking goes, I would rather not hassle with it. It takes a lot of time and patience to find where your computer is stable at. Another thing to consider is that Intel mobos are not to friendly towards overclocking either. A great overclocking mobo manufacturer to look at is DFI but if you were to go with a new mobo and cpu there would be no need.

Just checked in on Western Digitals website and they only have the 300gb model. I know on certain sites you can find OEM 150gb Velociraptors but I'd probably stick with the retail for the 5 year warranty.

If you like XP and you plan on getting more than 2gb of RAM go with the x64 version. I've been running it for a couple of years and haven't had too many troubles with it. Before that I was running Vista Ultimate x64 and had some problems here and there. There are pros and cons to both but overall I still like XP Pro x64 more.