this PSU would certainly be more than enough for the upgrades you're considering. in fact even if you had 2 6870's in crossfire, you could still probably get away with a quality 650W PSU. so then it becomes a question of how efficient you'd like (or need) your PSU to be. you wouldn't be drawing anywhere near 750W with the upgrades you're considering with the system at idle OR under load. if under load, a system like that would probably draw at least 20% of either a 650W or a 750W PSU's max rated output, and would therefore keep the PSU operating at at least 82% efficiency. at idle, the system would have to consume at least 130W (20% of 650W) on a 650W PSU to achieve a minimum of 82% efficiency. for a 750W PSU, the system would have to consume at least 150W (20% of 750W) to achieve a minimum of 82% efficiency.
yes, i suppose if your machine will spend most of its time requiring less than 100W @ 75% efficiency, its only drawing ~133W, which is still very little and relatively inexpensive power draw.
