Looks good, but I want to make a suggestion. Presuming that this is a top down look with your DVD being in the "front" of the xbox, you might want to consider rotating and swapping where your PSU and MOBO are.
Benefits:
1. Your video and LAN port will be rear facing. This makes for shorter connection to the LCD and easier access to the LAN port.
2. Your IDE cable will be considerably closer to the DVD/HDD, so less clutter.
3. The existing gap in the bottom center between the DVD and the a/v header will fit in nicely with the area you have designated for the HDD. May as well consolidate some space for future needs.
4. Your USB/controller ports will all be in the front, so shorter wires will be sufficient.
I couldn't make out the model of DVD drive you have, and can't see the version of the xbox...again, if it isn't 1.6, pick up
http://shop.sickmods.net/product/ATX_to ... nverter_2/" onclick="window.open(this.href);return false; and a pico PSU if it won't blow your budget. That could free up a lot of space in the ol' box and give you some more options for... well... more options. It would also give you enough space to wire in the power connector, and have your plug in located in the rear, which I belive is why you have the PSU located where it is.
Can't remember if we have gone over the PICO PSU and ATX converter before (sure we have) sorry about your account. Project is looking good, I hope it gets nailed down shortly.