it's too bad that there aren't any cheaper mobos out that support i7 - the price ranges on them are REALLY good.
in that thread, do you REALLY need quad-core for a sample farm, or would a really fast single processor (aka, e8x00) be good enough? don't forget that most rendering outputs are based off of single-thread calculations, and four cores at 2.4 won't be faster than two at 3 or higher.
you should look at prices for hard drives before you worry about cost. read this. i'd prefer you give me a price range rather than a size range, with costs being SO low for space nowadays. always better to go more than less.
with xp64? i'd go with a mid-level 8-series gpu. probably 8400 or 8500, they're really cheap and do the job for what you'd need.
mobo, nowadays it's rare (if you get a good one) to have issues with cpu support, like you had a while back. whatever cpu you'd get would likely be underclocked slightly to allow it to run in a smaller environment without an issue. gigabit lan is pretty much standard nowadays, too - even my atom board came with one, and that's intended for in-car dashboards =)
i'll pm you with pricing info based on what you've told me, but more specifics would definitely help.