one thing you missed is that you can recycle parts, ie case, PSU, which will save you a few hundred each time. Invest in a good PSU ( corsair TX series never goes a miss,) and case and it will last you for years.
Also you can sell off the indervidual parts when you choose to upgrade offsetting the costs of the new parts. I upgraded from a i7 920 to a 2700k about 6 months ago ( obviously change of ram mobo and cpu) and also from a gtx 480 ( which i managed to flogg for £200 when i bought it for £240 a year and a half before, because its somewhat of a modders dream card) to a gtx 670. So in all to get a totally up to date machine, which would have cost easily over £1k, i spent £450. Bargain aye?