1. I have not tried to run it without the washers on it, but I think those are there to avoid contact with the metal case, so you could have a static electricity issue if your case touches your mobo when powered. I am not certain of this though. I know it would probably be a pain to take your mobo totally off again to put the washers on.
2. DX10 is Vista only, but you can run DX10 on 32-bit Vista just fine. MS is saying the DX10 will NEVER be available for XP, but time will tell.
3. XP is much more intuative and you will get better gaming performance from XP, but Vista SP1 might change things. I will not "upgrade" to Vista for a while.
You can make your own washers from cardboard if you are concerned about it. Your DX10 card won't be wasted on XP.It will run DX9 games very well.Most DX10 graphics cards are having a tough time with DX10 titles.What graphics card do you have?