This might sound like a smart a$$ comment. If you finace one the before your done paying for it your machine will be outdated. Why not just build one that way when you can afford the part you can go out and get the newest or best one. And that way once it's built it's not outdated and then if you need to update the computer you can or you can sell it and start over.
 
I agree with Jgedeon. Once your done paing for It it will be out of date. Just save up and then buy a system. Mabey you don't need a new computer yet anyway, just make some tweeks.