I was not only looking at performance here but PSU as well the CM 500watt should be able to handle the HD6850 just fine but might struggle a bit with the 2x HD5750 which would probably give you more performance.
Cooler Master PSU's are a little hit and miss in the quality department!
Aha, I forgot about that.
After reading on what you've said, I checked the PSU and it looks like it lacks a power connector. Looks like CrossFire is not an option. Cooler Master products kinda 'rule' here. Not much of a choice. Recently there's Huntkey, Antec, Corsair, Gigabyte. The selection is not much and the price is quite expensive.
Do you have any suggestion to what PSU I should upgrade? I think I'll take up HD6850, and maybe CrossFire in the future.
The HD5750 really should do quite well at 1280x1024. If you are considering gaming on the higher res monitor then an upgrade would make more sense.
I tried and the difference is like, Heaven and Earth.