Hope this helps ==> 2x6800GS roughly = 2x6800GT, so...
all stock cards:
Far Cry - 1600x1200 4AA/16AF
6800GT SLI = 56.5
7800GTX 256mb = 48.2
X1800XT = 65.2
7800GTX 512mb = 64.1
Splinter Cell: CT - 1600x1200 4AA/16AF
6800GT SLI = 49.4
7800GTX 256mb = 41.5
X1800XT = 47.4
7800GTX 512mb stock = 55.1
HL2 - 1600x1200 4AA/16AF
6800GT SLI = 74.6
7800GTX 256mb = 82.8
X1800XT = 80.6
7800GTX 512mb stock = 88.2
Doom 3 - 1600x1200 4AA/16AF
6800GT SLI = 65.3
7800GTX 256mb = 51.9
X1800XT = 53.7
7800GTX 512mb stock = 67.7
Chronicles of Riddick - 1600x1200 4AA/16AF
6800GT SLI = 52.5
7800GTX 256mb = 51.0
X1800XT = 34.5
(ouch)
7800GTX 512mb stock = 70.0
FEAR - 1280x1024 4AA/16AF (Softshadows OFF)
6800GT SLI = 54.0
7800GTX 256mb = 43.0
(ouch)
X1800XT = 60.0
7800GTX 512mb stock = 53.0
Call of Duty - 1024x768 4AA/16AF
6800GT SLI = 44.1
7800GTX 256mb = 38.0
X1800XT = 43.6
7800GTX 512mb stock = 44.0
*Note: wow we really do need next generation cards.
3dMark 05 - 1024x768 noAA/noAF
6800GT SLI = 9438
7800GTX 256mb = 7767
X1800XT = 8821
7800GTX 512mb stock = 9335
But I have a slight problem with those scores since you are not stressing the cards and you'll be using AA/AF in the real world. Then,
3dMark05 - 1600x1200 4AA/16AF
6800GT SLI = 4697
7800GTX 256mb = 4764
X1800XT = 5522
7800GTX 512mb stock = 6105
These scores actually help to reflect reality better:
7800GTX 512mb > X1800XT > 7800GTX = 2x6800GTs
Anyways, I would get X1800XT from the link nitromullet posted. However, you might want to wait 2 more weeks for R580 if you are going to spend $500 on a graphics card setup because R580 for $499 will blow the doors off any of these options here.