It's much better to go for one good card than two weaker ones, just as many programs don't scale on multiple cores, some games don't scale well on multiple GPUs. And some even manage to do worse with multi-GPU setups.

It just seems like once you get into the $400-range, your RoI starts to plummet compared to a $200 card. Am I wrong?

Your Rol?

Either way, when it comes to multiple cards, you also have to keep in mind that it'll most likely use more power than one, generate a lot more heat, and you're also going to end up with half the VRAM you pay for going unused (two 1 GB cards will only yield 1 GB usable memory, not 2, for example).

Return on investment. "Bang for your buck", if you prefer

And I meant a single 200-dollar card in my last post

Oh, that's an *eye*, not an *ell*. >_>

Ultimately, it depends on the card, I guess. Generally speaking, though, there's always a premium on flagship models.

Right now, with the ATi 5000-series cards, there's a bit of a gap between ~170$ and ~350$, but there's a rumoured card launching in like two weeks that'll fall right between the two, which would probably be what you're after in that case.  There are other older options, but I'd personally go for the newest stuff, so that you get DX11 support and what not.

The "more power" build in the OP will give you the best high-end price:performance ratio for your money in terms of gaming, and probably only cost around $1200-$1300 with Windows 7 and a decent monitor. Anything more than that is just wasted money. Otherwise, if you want to save even more, you can buy mid-range + OS + monitor for about $1000 and put the rest of your money away for upgrades a couple of years down the line.

Yes. The boundary shifts a bit though, I'd say above a 5850 is poor ROI this time. And it always depends on what your performance needs are.

Just an FYI for those of you going the Athlon II X3 route: You may want to wait a few days before ordering. Today, AMD released, among other new CPUs, the Athlon II X3 440, which offers a slight clock-speed bump over the 435 (3.0 GHz vs 2.9) at the same price. It has yet to show up on Newegg, but when it does I'll update the OP.

Is there any reason to go higher than 4GB of RAM right now?

If you're just using your PC for gaming, then nope. The only reason that you'd need more than 4 GB is if you're running some sort of RAM-intensive scientific modeling software or something.