all the wattage rating means is that (for example) 750w is the max amount of power the psu can output. but if your computer only is using 400w at a given time, then that means its only outputting 400w... iow its fine
so if I'm planning to leave a door opened for upgrading cards or using crosshair/sli, I should get something with more wattage then ?e
That Antec NEO ECO 620C 620W PSU that shovenose recommended to you will have plenty of power for 2s HD 5770 video cards. Or enough power for one of the new next generation big video cards - think HD 5870 replacement. It's a very good choice.