Used FX1800s are selling for $240 on Amazon. A new one is $400+ at the Egg. That's a notch above entry-level for workstation cards. It ain't much at 3D gaming , maybe a 9600 GT or so pulling 60w.
At 19x10 resolution a Radeon HD6850 is around $150 and might
work without a new power supply (though it's always good to get one for an older system when you add new components) though you'd really be at the limit. A corsair 430w is $35 and should take care of things nicely - hope it fits in that Dell case
I'll guess the HD6850 is roughly the same size of the FX card so it should fit -- unless you have some odd-ball setup with the super
-slim Dell case.
And an HD6850 will slobber-knock anything you could get from a 9600gt-like card and should work great with your CPU. You should be excited.
If you are gaming at a much lower resolution, something like the HD5770/6770s are in the $100 range, and would still p'own that FX badly in 3D gaming.