Pay the extra $19 and get the above, the reason i say this rather than the X4 is that the X6 have more longevity in them, and will out perform the X4 as we start to see better multi-threaded enable games and software released as we now are with titles such as Battlefield 3. Also if you wish to OC the CPU at a later date then you can with the addition of a decent aftermarket cooler like the HYPER212 plus. This CPU will normaly reach 4.0ghz on air quite easily, which makes for a very quick system indeed, trust me i have one and talk from experience.
Both will work just fine for current gen games, the more expensive x6 will work better for yet to be released future games, but when that time gets here you may need a whole new core anyway regardless of spending now on the x6.
600w will do either processor and the 550 just fine, but only if it is a good brand. Antec, Seasonic, XFX, and Corsair are the best brands, most of the rest just suck.