Asus P5E Deluxe + Q9450 + what speed RAM? Help!

I haven't purchased RAM since the biggest question out there was "will PC133 work on my 100 mhz FSB motherboard" and have been reading the forum here, FAQ's for two days now trying to bring myself up to speed and am admittedly quite frustrated as it seems the world of RAM has become extremely complicated and I feel no further ahead than I did when I started. The FAQ in this forum is outdated as are most of the posts I find.

Can someone please help?

I have decided on an Asus P5E Deluxe motherboard and the Q9450 processor. My understanding is that this processor has a FSB of 1333mhz , and that I would set the P5ED's FSB to 1333mhz to match. I am also under the understanding that the memory bus can be set for either DDR2 800 or 1066 with these settings but my questions is, which is preferable? I have narrowed my choices down to these two and don't care about which one is more expensive - what I care about is which on is the most "compatible" with this MB/CPU combo.

Is there a good, current link out there that can help me (an others) understand how to make good choices when selecting RAM for specific MB/CPU combos?

I am also wondering - if I but 4GB of ram in this system and install XP Pro, will it only recgonize 3 GB? If so I'm OK with that (I run Pro Tools 7.3.1 which is incompatible with Vista and x64 http://www.digidesign.com/index.cf [...] emid=26406), and don't mind XP only being able to use 2.75-3 GB until I am able to upgrade in the future, but am wondering is there any downside to this config? In other words, is it better to only physically have 2GB (2x1) DDR2 instead of 4GB (2x2) or is there absolutely no difference to the system or XP besides the size difference?