I always recommend to people, save your money and buy all of your parts at once, for one key reason.
Most companies have strict return or RMA policies. If you buy a motherboard, and have it for 6 months while you buy the rest of the parts, then go to build your pc and the mobo is a dud, you now cannot return the mobo due to the RMA time frame having ended.
While it is tempting to buy things as you get the money, avoid the headache and just save until you can afford it all at once.
My .02
I did decide to do it your way sort of, I bought half a while ago and now finally I have enough money for the rest of the parts. I just underestimated the pricing when I first went searching for what I wanted.