just start grabbing the essentials... power supply, ram, HD...

leave anything that fluxuates month by month for the last of the purchases...

Unless you are looking at SSD's, you should be able to get your power, case, ram, hard disk(s), mobo(?) without commiting on a finished product.

Last things would be the processor and the video card... unless your using an all in fucked over gpu.

But if you find a good deal, grab that shit.