I think you meant x1900xt, since the x1800 is actually cheaper than the GT. And the x19 is only about $130 more. Not $200. So another $170 on top of that will get you a second GT. I wouldn't call that "almost". Either way, for running today's (and tomorrow's) games at 1680x1050, the OP will be better off with an x1900xt or SLI-GTs, instead of a single GT with dual-core.
I stand corrected, I did mean 1900xt, the $200 more is what the op said.
Why is it getting old? Because it's true? Please list these "more and more applications" that actually get a significant boost from dual-core and the OP can tell us if he actually uses those applications. So far, the only ones he's listed will get nothing from dual-core.
I have used dual cpu's on my work machine for years now, it's just much more responsive despite none of the apps being coded for it. Now the apps are starting to be coded for it - e.g if you look at say office 12 (the next microsoft office) that starts off running 6 threads, do something and you're up to about 10 - multicore is just going to be come more and more important.
So, using the ONLY game that shows ANY performance increase with dual-core, you get almost 100% increase at the OP's playing resolution with 2 gfx cards vs. 0% with dual-core CPU... for around the same total price. Please explain to me again why dual-core is worth it from a price/performance perspective...
All depends where you're bottle neck is - you want a balanced system. Remember you can dial down the graphics options - Quake 4 will run at high fps with a lot less powerful graphics cards if you don't run at max settings and still look nearly as good but there's a lot less you can do if your cpu isn't up to the task.