Currently I have 'normal' gtx 480, now i am going for SLI , so I have to decide between Zotac GTX 480 AMP and MSI GTX 480 Lightning for second card. Reviews show that GTX 480 AMP consumes less power than GTX 480 Lightning. And currently i am low on power 'cause currently I have Corsair TX 850w power supply. So i have to go for a card that draws less power. Other Forums tell that TX 850W is more than enough for Dual GTX 480 unless u do overclocking of any component.
My current Configuration,
i7-920 2.67 ghz
Corsair 6 x 2 gb ddr3 1600 Mhz
Asus P6T deluxe V2
3 x 1 TB western digital Caviar green
Point Of View GTX 480
Palit GTS 250
Cooler Master Haf-932 cabinet
If I were you I would go with the Zotac. When I was researching what fermi card to get I read horror stories about MSI. Of course we are talking about the internet and anyone can say anything when they are pissed off. I personally have never tried MSI but after reading some reviews on various websites I don't think I want to try my fate.
This is a very tough choice. The Zotac has the Zalman cooler and has been out for a long time. The MSI Lightning has the Twin Frozr III cooler and was just released. I think the key might be the amount of space in your case for SLI. The Zotac is a 3 slot card, and the MSI is a 2 slot card.
It looks like the MSI Lightning is your card: It is smaller for better airflow in your case, it is MUCH quieter, it has better performance, and has a higher quality build. The main positive for the Zotac card is lower power consumption.
I think your 850w Corsair PSU is fine, even with overclocking. That is a benefit from owning a quality power supply, it can handle more than its rating. 850 watts is the minimum recommended for GTX480's in SLI.