Just some numbers to compare my old system with the new one (coming soon!) :

Old system : Pentium III 866EB w/512mb RAM & nVidia GeForce 4 Ti 4600 video card.
New System : AMD FX-55 w/2gb RAM & dual nVidia 6800 Ultra video cards in SLI. (SLI just means dual video cards, one for the top half of the screen, one for the bottom half)

In Doom 3 at 1280x1024, with 32 bit color:
P866EB = 24.5 fps
FX-55 = 96.8 fps

So the new CPU alone is roughly 4x faster as far as playing Doom 3.

Using a standard benchmarking program called 3dMark2004 :
Ti 4600 = 2092
6800 Ultra = 11873 (note that this is ONE 6800 Ultra)

So a single 6800 card is six times faster than my old video card.
Now we go a bit deeper.

They updated the benchmark program for the newer cards, now it is 3dMark2005 :
6800 Ultra (one card) : 5196
Two 6800 Ultra in SLI : 9382

So the SLI combination of two of these cards is nearly twice as fast as a single card.

Doom 3, in High Quality, at 1600x1200 with 32bit color, 4xAA, 8xAF :
One Ultra 6800 = 42.8 fps
SLI Ultra 6800 = 73.4 fps

Wow! Speeeeeeedy! From my own testing with the above settings on my current machine, I get between 1 and 4 fps depending on if I am looking into a room or staring at a blank wall. My system simply cannot handle Doom 3 at high quality in high resolution mode. I was getting 30-40fps when I dropped resolution to 1024x768 at medium quality. Playable, barely, as it dropped to 15-20fps in some larger rooms or in heavy combat.