Preparing for Crossfire - advice on PSU, airflow

Hi Tom's,

My current rig's about 18 months old :

i5 2500K @ 4.3 GHz (air, with a CM Hyper 212 cooler)
MSI 6950 Twin Frozr II, shader unlock, 880/1375
Biostar TP67XE mobo
8Gb Ripjaws-X RAM
60GB OCZ Agility 3 SSD (Windows)
1TB HD (storage)
250Gb HD (Ubuntu)

It's served me well so far, bar some bluescreens that flashing the SSD's firmware seemed to fix. The 6950 chugs a bit when it comes to BF3, Arkham City and ARMA II on top settings, much to my annoyance, even though I don't actually play those games a whole lot (I'm sure you guys will understand), so I decided to treat myself to another 6950 (the Twin Frozr III model) on one rare occasion when I found a reasonably priced one on eBay. I'm hoping that one will unlock as well, and ramp up to the same 6970 clocks as my current card.

After trawling several forums it seems like the jury's still out on whether my Cooler Master GX 650W (Bronze) has enough juice to power it all. From what I gather, some think it'll be enough and that recommended wattages tend to be a bit OTT, while some will recommend I get at least a 750W for peace of mind, and both camps seem to argue their points equally convincingly.

Will I definitely need that extra wattage? Add a new 750W+ PSU and it starts to look like an expensive upgrade (I'd probably go for something like the OCZ ZS series), though still cheaper than a brand new 7970, which I gather I'll see similar performance to with two unlocked/overclocked 6950s. Will 750W even be enough given the overclock on the CPU and both cards? Am I going to end up spending close to the price of a 7970 on it after all?

One last thing - although temps are fine (CPU/GPU hit about 65-70C under stress with Heaven/Kombustor benchmarks, and it's not like I'll be running them all day long), would you recommend sticking a couple more case fans into my Cooler Master CM 690 II? There's a slot on the side just by where the cards will be. Will I need to balance this out with an extra exhaust fan? I'm not too familiar with the vagaries of airflow...

Thanks!