It can probably be done better, but just goes to show, that its easy enough to get something going for less than $300, and this is even a halfway decent looking case.
Of course, you'll also need an OS which in the case of Windows adds at least another $90 or so for an OEM version of Vista home Basic, bumping it up to $369 total... Plus shipping for all the individual components.
Nowadays building your own is pretty much always more expensive than buying pre-build, thanks to the volume discounts by the OEM's. for example, for $349 you can buy a Dell Inspiron 530, with the same specs as Zawran's list, plus Vista Home Basic, a 16x DVD-RW drive, Keyboard, optical mouse, MS Works 9.0, and one year 24/7 telephone support and in-home warranty service.
Shipping is $29.99, which is probably cheaper than ordering all the parts seperately elsewhere.
Now, I'm not saying that you should get a Dell or any other brand, just that price-wise, building your own isn't always going to make it cheaper than buying a pre-build machine, especially if you also need the operating system itself...
From a warranty point of view building a pre-build can also be easier: it's a single solution, and if it doesn't work they can't really blame someone else if it doesn't work... None of those "the memory isn't 100% reliable in motherboard -x-" kind of issues.
the big advantage of shopping for seperate components is that you can get exactly what you want, e.g. a specific brand or size of harddrive... But from a purely economical point of view pre-built tends to beat home-built these days.