i was thinking about doing something like this since i build all my own desktops, but have yet to do so. the only thing i would worry about is special drivers and such for special laptop functions such as function keys, etc. this was also caused my hesitation to run linux on my current laptop, but it works fine and isnt causing any probs. i guess what im trying to say is that i worry about some laptop specific functions.
further than that tho, i think its a good idea. mose of your boutique laptop makers use models like this to build their laptops. often times you will see sager's, alienwares, voodoo pc's, etc. all look the same because they take chassis like the one your looking at and drop other components in it.
i like your choices; MSI is a good company IMHO, i am running a couple of their mobos right now. im not sure if you meant a diff processor tho, coz a single core athlon 64 is kinda old school, but i assume you mean a x2 or maybe a quad core? my only gripe with your setup would be your video card, i would go nVidia, but thats just my opinion. to each their own.
let us know what you decide to do.