I would consider the 750 watt power supply as a minimun for a SLI rig with a top-tier card. The 850 watt unit would be my choice for a high-end SLI rig.
Newegg.com - CORSAIR CMPSU-850HX 850W ATX12V 2.3 / EPS12V 2.91 80 PLUS SILVER Certified Modular Active PFC Power Supply - Power Supplies
If your considering Tri or Quad SLI then you should be looking for a 1000 watt minimun.
Newegg.com - CORSAIR CMPSU-1000HX 1000W ATX12V 2.2 / EPS12V 2.91 SLI Ready CrossFire Ready 80 PLUS Certified Modular Active PFC Compatible with Core i7 Power Supply - Power Supplies
Personally I would want more headroom for Tri or Quad SLI but finding a Modular Power Supply better than the one I haved linked will be hard to do. Personally, I wouldn't be considering a Modular unit in anything high-performance, or at least not until they become more popular and more options are available.
I've been using a Cooler Master 1250 watt PS for about 3 years now and I haven't had any problems. It's only $10 more than the 1000 watt Modular unit and it's 250 watts bigger
Newegg.com - COOLER MASTER Real Power Pro RS-C50-EMBA-D2 1250W ATX Form Factor 12V V2.3 / SSI standard EPS 12V V2.92 SLI Ready CrossFire Ready 80 PLUS Certified Active PFC Power Supply - Power Supplies