well to start with, is it Mini ITX, or Micro ATX, coz as far as i know there is no Mini ATX? I could be and probably am wrong tho
Do u really need the video card? most of these smaller boards have onboard video, it might be a good idea to stick with that to save on some space?
If you're going for an older P4, you might find you need DDR1, not DDR2.
Also, not sure about others opinions on this, but you might find it better to go with LESS ram. If you are intending on using hibernation, the system has to read/write that full 2gb of ram to/from the hard drive during startup and shutdown. I'm at the same stage you are, picking bits for my setup, and i'm going for 512mb and no more for that very reason.
I'm also going for a 3.5" to 2.5" HDD Converter - http://www.dealextreme.com/details.dx/sku.727 . If you have your laptop harddrive in an external case (USB?): 1/ you'll have to make sure u'r motherboard supports booting from a usb drive. 2/ It might be a tad faster to use the adapter and go from IDE and not usb.
One more thing, if you do ditch the graphics card, a PCI Riser might keep everything at a lower profile with the sound card. They're basically just a pci slot that plugs into a pci slot at 90 degrees. Type "pci riser" into ebay to see what i mean, they're pretty cheap. Also look for a "reverse pci riser" see which direction would suit your setup best.
Good luck with it anyway, hope it all comes together