woooooo.
I could be wrong, but I think it is very important to NOT put SP3 on right away. Many of the Media Center updates will not install properly if you put SP3 on. There is a thread about it all somewhere here, but I can't find it right off hand. Essentially it was to install MCE2005 using your 2-disk round-robin install. Tell it NOT to do automatic updates yet. THen install .NET 1.1 and .NET1.1 SP1, and one of the .NET critcal updates. Get the two Media Center updates, ROllup 2 and the last cumulative update for media center. Then after you have gotten those two updates you can let it do auto updates, I think. In my opinion, I would let it do it in this order, or go find a better listing. I know the one time I saw a post about this there was a semi-official MS answer that detailed the steps very well.
MAIN:Gigabyte EP45-UD3P, E8500 C2D, Silverstone LC16B-M case, 3xAverMedia M780, VistaView Saber DA-1N1-e, Vista Ultimate TV Pack.
Bedroom: HP SL4282N Media Center Extender TV
Basement: P35-DS3R, E4400 C2D, ATI3650, VBox DTA 150