New Vegas does not utilize crossfire for ati cards.

It only uses 1 gpu on the 5970 card, but I've found a fix which runs well enough to almost not warrant the need of an actual official patch.

First things first:
1) Download ATI CrossFire Xtension - Found at http://www.mediafire.com/?iz5mzzzz50z
2) *Optional* - Scan that thing at http://www.virustotal.com
3) Add a new profile
4) Create an app name, also link the app the your FalloutNV.exe
5) Only enable Force CrossFire Support, select Alternate Frame Rendering in the Method, select Direct3D and set the pre-defined profile to FallOut3.exe

Ok almost there.
If you've already tried d3d dll file fix, you'll notice it stabilizes framerate at the expense of max framerate
However, when it's used in conjunction with the forced crossfire, the second GPU makes up for that loss.

6) So download the old d3d dll file at http://www.newvegasnexus.com/downloads/file.php?id=34778
7) Put it in the same folder as your FalloutNV.exe
8) Launch New Vegas from the ATI CrossFire Xtension


Measure the framerate differences BEFORE the d3d dll file and Forced Xfire using Fraps (http://www.fraps.com/)
Then test with this "fix" to see if you gain performance.

*Additional Things to do to improve framerate*
9) -Using the New Vegas launcher's options, set Water Multisampling to LOW as this feature seems to be bugged
10) -While in game, hit your tilde key (~), and type in ToggleEmotions - This turns off NPC emotions which also seems to be bugged since it causes a massive CPU drain.


Post here if it works for you, maybe this can be of use to more than just me.

I tried the Crossfire Xtension 1.4 yesterday without the d3d9.dll and it didn't enable crossfire. I tried d3d9.dll without Xtension and it didn't boost my frames but it did enable crossfire.

I'd like to try and force it thru Radeonpro but it won't run on mine and I don't feel like finding out why. There is a set of drivers at http://forums.guru3d.com/showthread.php?t=322616 which http://www.gamesas.com/index.php?act=findpost&pid=16550021 they had luck with to enable crossfire. I remember them saying they were using these drivers and playing the game, and as soon as they upgraded to the official 10.10 drivers their fps were cut down a lot and the game became choppy.

I used the d3d9.dll (106,496 bytes) with the same .ini I use for normal play (tweaked for extra detail) with only the sD3DDevice being different. Aside from the perspective being a bit different and shadows being lighter with the hacked d3d9 present, visuals looked the exact same but there was no benefit framerate wise, it may have even been more choppy.

The game is playable with the 5970 on one GPU but the fps go low sometimes, especially when I have my draw distances for everything increased dramatically. I'll just deal with it for a while longer. My main concern right now is figuring out why all dialog sounds like I'm in a small room with reverb even if I'm outside.

[edit] fixed reverb by setting bEnableEnviroEffectsOnPC to 0 in fallout.ini

ATI version: http://www.newvegasnexus.com/downloads/file.php?id=34970
The normal one breaks transparency multisampling/vegetation.

The ATI centered version does indeed improve performance and graphics over the first dll posted.

I'm glad I changed them...

19x12
4AA
16AF
V-sync on
Ultra

60 fps...

With the other version it was 50 to 60 with loss of graphic quality and only 2AA and still a few stutters.

Now it flies..!!!

I have a 5970

Is using the .dll the only thing which made it fly?

OMG awesome, I put this version of the DLL in and crossfire works, for some reason it enabled vsync or that ipresentinterval=1 on, so I set that to 0 and set the ini files to read only so it wont change back.

Now I have 2560x1600, 16xAF, 2xAA, Ultra settings (all max)
About 60-80 FPS
No mouse lag because of vsync
No stutter around NPCs

Awesome find!