crysis900 :
There is definitely something wrong, I run Crysis on my 9800GX2 KO all settings Very High (Shaders high) and get about 55fps at 1280X1024. I get similar results with Warhead. Get the latest drivers on Nvidia.com.
^^ Same with me. At 1280x1024 at least, a GX2 should get in the mid-50's with everything on, no AA (note, I operate XP, so I'm probably inflating a bit...). I find running Crysis at above 1280x1024 just stupid at this point anyway.
For the time being, use the latest betas (180.42/43), as they seem stable enough. The 180 series of drivers give the GX2 a bit more attention driver wise, although the card should do Crysis fine by itself...
Once the drivers are in, get into the NVIDIA control panel, and make sure multi-GPU mode is set to on.
Note the OP said that he could tell the diffrence in non-Crysis games, so I'm chalking this one up to Crysis being Crysis. You have to realise the GX2 starts to stuggle at 1980x due to its lack of RAM. It should be running faster in Crysis than you are describing though...
As for heat, I used rivatuner to set the fan speed to 80% constant, which keeps the card below 75C max (note, that Rivatuner gives the option for two fans, one for each card. Only one of these actually does anything, so set 1 of them to 100% to see which one is the actual fan controller).