My first 'real' bicycle purchase is pending by saving ability. I've been considering ordering a bike from Bikes Direct, but recently by attention has been caught by some affordable frames and forks on Nashbar. Of course I understand I'll need literally everything if I build my own, and either way I'm headed to the closest coop for assembly help.

I know what sort of fit geometry I'm looking for, and can achieve this with either option. Otherwise I'm completely inexperienced with what I'm actually doing with mechanics work (strike one against self-assembly, except maybe not since I'll have at least reasonably knowledgeable help ... erm, instruction).

As far as budget is concerned, I think I might be able to come out a bit ahead with the frame-up build, with some budget parts. But then it already gives me the upgrade bug, thinking why not get something better than just somewhat serviceable? That'll blow the budget, and I'm not sure how I'll feel in another several months when I'm replacing my second chain, and need a new rear cassette, and feel like I'm just re-doing work I should have done in the first place (or perhaps using parts maybe I should have).

If I go with the Bikes Direct option, I intend to eventually replace the drive train, and perhaps more, as time progresses.

But as saving is slow right now (I REALLY need a better job lawl) I have time to make these decisions. I'd much rather have advice from more experienced riders than myself.

Go with the whole bike. In my experience, I have bought three brand new bikes. The first one (ten speed road bike) was stolen after about two years of ownership. The second bike (12 speed roadie) lasted for around 12 years till I gave it to a friend after buying my current bike. I have over 18 years of ownership with the current bike (hybrid 700c) and I haven't changed a thing except for the front wheel, rear tire, brake pads, and the seat/seatpost. It really needs attention in the area of the drivetrain but this is after owning it for a long time. It owes me nothing now and I sometimes think about spiffing it up with a new drivetrain, shifters, cranks, etc. Then again I can buy a new bike and be done with it for a long, long, time.
You should get lots of use from a new bike before you need to replace the hard stuff. Just keep her clean and dry!