Add HD 6950 in addition to 9800 GT (say to number crunch with everything), or replace 9800 GT with HD 6950? Overclocked 8 core Bulldozer with unlocked HD 6950 and several hard drives shouldn't break 550W in the worst case scenario. It's not like a Seasonic X Series unit is going to explode if you reach the rated continuous wattage, though it may become a little loud.
If you are really going to be stressing it with near max loads all the time, or would prefer lower noise in those types of conditions--note: max loads don't happen with gaming, and your graphics card will probably be noisier if being run that hard--or maybe would upgrade to something more power hungry (keep in mind that power consumption is on the way down), then the X-660 makes more sense.
But since it's 7 euros, not a big deal either way. As I mentioned, get something other than Seasonic X Series if you want a great unit at a better price.