You seem to have an excellent system now. Unless you want to have two complete gaming systems around to use, I would just spend the money to upgrade your graphics subsystem.
I would rate the FX-60 superior to the Pentium D 3.2GHz EE for a couple reasons (I assume you're talking about the Pentium EE 840). First of all, I don't own a FX-60, but I want to make a general statement about AMD vs. Intel processors. It's pretty much widely known that Intel 8X0 series has a power issue-high leakage currents-which is why the processor line is limited to 2x 3.2GHz only. Since the 840EE probably consumes much more power, it'll probably require a louder fan to cool than the FX-60. When I'm playing a game, I don't like to hear the processor fan rev up too loud. If you really want to spend over a thousand dollars on an Intel processor, why not go for the new 955EE anyway? It has a higher clock speed and, since it's made with a 65nm process, is pretty overclockable (so I've read).
As for Dell PC's, I find that you can find retail parts and build the same system for cheaper than they'll sell it to you. The only problem is that Dell probably has a higher shipping priority for the GTX 512 then say Newegg or some other E-tailer.
As I said earlier, I don't own a FX-60 or any Intel dual core, but from what I've read from other threads and THG articles, the FX-60 is an excellent processor which competes with Intel's 9XX series dual core. Since many games offload work to the processor, I would imagine you would see a performance DROP even with the GTX 512's.
I've never bought a Dell gaming PC, andI don't know what brand of memory they use. If you have 2GB of good ram, CL 2 or 2.5, then you might get stuck with CL3 in your Dell (I have 2GB of Samsung CL3 which I got in an Dell Optiplex GX270 system).
If you're comfortable working inside a PC, I would just try and upgrade your graphics subsystem only. I know its hard to find/get a single GTX 512 in online stores, so it might be hard to get two of them. I hope this info is helpful. Good luck.
EDIT: Here's the THG article which reviews the FX-60. You can see both the FX-60 and the 840EE in the benchmark results.
http://www.tomshardware.com/2006/01/10/amd_athlon_fx_60_dual_core_assault/
2nd EDIT: Here's the THG article which reviews the Pentium D in general.
http://www.tomshardware.com/2005/04/05/the_pentium_d/