The G series is Seasonic's new gold-rated series, they're good units. The 550W unit you mentioned is indeed on sale for $60 AR, go ahead and grab if it it's for a gaming system. It's both highly efficient and modular and comes with 5 year warranty and is great quality all-round, you won't be disappointed.
S12II and M12II are mostly older units, although good quality and worth buying if they hit the same price per watt as the G550 now. They're all 80+ bronze so they're less efficient than G series, and only the M12II are modular. Then there's the X series which is Seasonic's high end and contains fully modular 80+ Gold and 80+ Platinum units that are typically very expensive and not worth it for most people.
Last edited by lehtv; 11-14-2012 at 06:48 PM.