I just asked this question a few weeks back and just went through a major upgrade. I learned quite a bit in the process.
What you get depends on several factors. For example, as you mentioned, if you are running a 32-bit OS then you are limited to 3 GB of RAM. Therefore you won't need a motherboard that supports 16GB. However, if you are planning on going to either XP 64-bit or Vista 64-bit, then you might want to plan ahead and get a motherboard that will allow you to put a lot more RAM in it.
The Asus P5K motherboard is a great motherboard, but there are a few cautions. One is that the board is about a year old. As a result, so is the technology. I had this board in my machine about two days ago and ran into a problem. I have Vista 64-bit installed and put 8 gigs of RAM in (this board supports up to 8 gigs). However, I could never boot into Windows with 8 gigs. It would only boot in with 6. While the board itself was great, it limited me in this way. If you were ever planning on going to 8 GB of RAM then you would need to be aware of this possible problem.
The Asus P5Q, on the other hand, is a much newer and better board. It will support up to 16 GB of RAM with no problems. This is the board I am running at the moment. Currently I only have 8 GB installed, but it runs great.
Both boards are great (the P5K and the P5Q), but there are a couple of other differences to note. The P5K is an SLI board with slots for two (or three?) PCI-E cards to be run via SLI. As a result, the board has an Nvidia chipset. The P5Q is NOT an SLI board and has an Intel chipset. If you are planning on running SLI at some time in the near future (i.e. running more than one video card working together via SLI) then you don't want to get the P5Q. However, if SLI is not an issue for you, then the P5Q is a great board.
One other point of difference is the P5Q is ready for the 45nm CPUs (i.e. the Q9500, etc). So it supports both the 65nm and 45nm chips. Therefore the P5Q allows you to easily upgrade the CPU in the future should you want to. I believe the P5K only supports the 65nm CPUs. You could run the Q6600 on either board, but you would not be able to upgrade to a 45nm CPU on the P5K.
As far as your power supply, what you get will depend on several factors, especially your video card. I purchased one of those Nvidia 9800 GX2 cards and a 500 watt power supply was not really enough. I currently have 850 watts. Getting a big power supply won't hurt and it will allow you to easily upgrade other components in your system should you need to (i.e. bigger video cards, faster RAM, etc). If the power supply is too small then it can cause a slow down in your system (RAM can run more slowly, etc). So getting a decent power supply is a must. I would think that 500 watts is a minimum these days.
the Q6600 is a great CPU and they say it can be overclocked very nicely. It has a nice cache (8 mb) but is a bit slower than some of the 45nm CPUs. However, my previous CPU was a PIV 3.2 GHz and I can safely say that the Q6600 (my current CPU) is blazingly fast in comparison. I am very happy with it. Get a good heat sink to keep it cool. In fact, fans for everything (even your video card) and your system will run well.
As far as RAM is concerned: RAM is fairly cheap these days. If you are running a 32-bit OS then go ahead and get 3 GB instead of two. Why not? It will make a difference. If you get the P5K then it can take up to 2GB sticks and has four DIMM slots (pay attention to the problem I noted above, though). You can run three sticks of 1 GB each without a problem with this board. Two sticks will work, but with the fairly low price of RAM, why not add the extra stick?
I cannot talk about the video card you have selected because I have been an Nvidia user since the beginning and have never had an ATI card. I have heard good things about that card, though. The 9800 GX2 that I have is a great card because it is basically two cards in one (two GPUs and 1 GB of RAM). As a result, I was able to easily move around over 9 million polygons in my 3D application with my current set up while I was capping off around 250,000 with my previous one. The 9800 GX2 allows you to run in SLI mode (i.e. both chips and RAM powering one monitor) or separate them to run two monitors. I tried separating them to see how much slower things were in my 3D app and found that I was getting slow downs at about 2 million polygons. Somehow this card is significantly faster (2 million vs. 9 million) when both chips work together. I am telling you this because, whatever video card you get, do your research because it can drastically affect realtime 3D performance.
Well, I hope this has helped somehow.
