$400? get an emachine and when you can afford it, add a stick of ram and your choice of new video card.
Or maybe you have enough leftover components that you're just looking for a processor and motherboard? If you've got leftover components or a current machine that is new enough to scrap, or parts from friends or something, you could maybe swing it.
If you already have a case and some other stuff here's a scenario.
If you need JUST CPU/mobo
AMD Athlon 64 3500+ Venice
That's only $260 If you already have a decent AGP video card you can use, there's still a little room if you need some memory.
CORSAIR ValueSelect 1GB (2 x 512MB)
So that's still only $337
If you have enough leftover components you might be able to put something together. Maybe not entirely new, but newish.
My advice is to look around and see what you already have that you can use and figure out what you really need and don't need. If you put together something with a decent start and some old parts, you can always upgrade some things later.