Nice job on the build.. wish I had the patience to keep the inside of my media centre that tidy.
One thing. Why overclock the ram? Over clocked ram produces a lot more heat which as you well know is the enemy of a media centre. It is not like you need the performance.
I just replaced the northbridge heatsink on my MC. Was a Zalman ZM-NB47J but the northbridge was getting very hot. Just put in a Thermalright HR-05 Fanless which is SO much better. Watching blu ray on a hot day northbridge temp only got to 40deg
One thing with the video. The problem I am having is that the card is passive and the heat sink is all on the chip side of the card. There is not much air flow over that side of the card so it does get hot. Trying to work out some way to get the heat on the CPU side so that it can be taken way by the case fans. The are some VGA coolers that will do it but they are not cheap. Which it had a cooler like my old 6600GT did which had heat pipes to take the heat to the other side.
Anyway good luck.