Thank you once again for your help Justin.
Based on the fact that this is a lab environment and hence I try to stay up to date with the latest firmware, versions etc (the whole point of a lab is to play with technology and try new things), I'll probably forgo the ConnectX cards and look for the ConnectX-2 cards instead, provided they aren't too expensive. It also means that when the next iteration of ESXi comes out, they will be supported (hopefully).
In saying that though, the price point on the older ConnectX cards can't be understated.
I'm going to go away and try and understand the ConnectX-2 range of products a little better (including VPI), and no doubt I'll have more questions.
Thanks again.