Yeah there is something seriously wrong if you can't max out everything completely. ESPECIALLY in L4D2. You should be getting framerates in the hundreds. Sounds like power, memory, or CPU problems to me.
Adding another GPU to a bottlenecked setup isn't going to fix anything, only cause more problems.
There is also something wrong with your setup if you can't max out all games at 1920x1200 with two 6950 2GB's. Especially overclocked. Sounds like you might be having some issues also.
Maybe you could elaborate on what games you are having trouble with? We could possibly help troubleshoot why you have trouble maxing games. My crossfire 6970 setup makes short work of literally everything. Even in 5760x1080, which for just a couple games requires lowering some AA etc.
Even when I used just one unlocked 6950 2GB overclocked it pretty much maxed out every single title including metro 2033, and most games WITH fraps recording.
uh lets just leave it at i have far more knowledge on some games then i should
ever try shogun 2? dx11 max settings it uses nearly 4000mb of Vram, oh and i use modded unit sizes so instead of the shitty 2000 vs 2000 man armies the normal battles i fight are around 4000-8000 vs 4000 - 8000 so 8k to 16k soldiers on screen dx11 etc its enough to bring any system to its knees.
played Metro 2033 lately do you keep everything enabled you probably dont, but if you do and Depth of Field is on along with 4xAA frame rate is well below 60 lol like 35-40 avg drops down to 10-12
for testing eyecandy and generally just making friends drop there jaws,
Crysis with extreme settings and IQ mods your getting below 60fps easy
my judgement of playable frame rates and gameplay is far different then most on here. Ive had 4870x2 switched to 5850 xfire 6970 xfire and down to 6950 xfire do to heat reasons in all honesty with mods alone you can make gtx 580 sli or 6970s in die in Oblivion with relative ease if you push modifications and certain INI settings to improve image quality. I dont just play games i MOD them,
I dont tend to accept the base game as good enough if theres a way to improve the visuals ill do it lol
if Sandybridge wasnt limited to x8 x8 xfire or sli id have 3x 6950s at this point,but any board saying it has more then x8 x8 is a lie so its worthless purchase unless i went with a hot and loud 6990 but i digress what i see as good enough is probably far different then what you would consider playable,
a good example in Bad Company 2
i actually do better in game at 120 fps then 60 and my sppm and K/D scales with frame rate from 60 to 80 to 100 to 120 i do better at each step up i level off at 120 without further improvement, so while games might be considered playable by some they are not playable to me thus why i have always had 2x GPUs from the get go,
as for the GPUs nothing wrong with them Bad Company 2 averages 155 fps the 6970s got 170 avg so less shaders and same clocks its about right where it should be
as for Super XP a single 6970 is fine with a Phenom II problems start when you add 2x gpus to a phenom II system, unless that Phenom II is an X6 its gonna bottleneck hard, and even Erocker will tell you the x6 still dosent give optimal performance compared to even a 2500k lol comes down to system memory bandwidth, but yea single gpu he shouldnt see any issues with L4D2
but i digress at this point we are seriously off topic. lets get back on topic? before erocker starts hitting people.