I bought my desktop parts in separate chunks. Simply because I didn't have enough money at the time for the final total cost. Otherwise I would have bought it all together, nothing like the UPS guy dropping off 5+ boxes at your front door
When I did my first build a few years ago I bought as much as I could at once, but also bought from multiple places so it ended up still being awhile until all the parts arrived.
I now like to order a part when I can or know I am going to use it, because if I wait then I will see something else slightly better and then want to wait for it, etc. Though if you are going to be buying over a long period of time it might be best to just wait until you save up a bit.
Ditto. I watched the hot deals sites for a month or two and probably saved about the same cost. I was able to get a DVD burner free after rebate, free shipping on a case and some other discounts that added up quickly.
Generally speaking, to ensure the lowest possible price, you need to buy all the major parts at once (cpu, mobo, ram, video, etc). The other components like dvd-rw and sound card typically have a steady price.
RAM has a volatile price, so if you see a good deal (like the Ballistix for $60, no rebate, from buy.com), go for it. If you're not in a hurry, cash in on deals like that and you'll save a pretty penny. If you're in need of a system rather immediately, buy it all at once.