You'll probably want to read these:
- MHGH28-XTC firmware 2.9.1000 non-functional with ESXi 5.1?
- Re: Will there be ESXi 5.x driver support for ConnectX adapters?
If your budget can stretch to the ConnectX-2 cards without too much hassle, that might be safer.
The ConnectX-1 adapters can be made to work. But there are some limitations and the Mellanox guys consider it "not supported". (still, several people on this site seem to be using the ConnectX-1 adapters with ESXi 5.1)
