Since the GPU is the same, more Speed Performance comes from higher clock rates for the chip and its memory.
These are spelled out in the specs. The brand that happens to clock the fastest wins the speed game.
Some brands make their own boards with better quality components than the reference boards.
This does not make them faster, but as with motherboards, it can allow for more stable overclocking and longer life.
Most brands try and differentiate by using their own style of GPU cooler.
Some are quieter than others. Some cooler, and some both.
For any specific GPU class, (in your case 6850), reading reviews on competing cards is the only way to learn why some cost more on the market than others and why.
I would be looking for deals on 7000 series cards, but that's just me.
Oh and about that no-name 500 watt PSU.
In an unstressed environment, these boxes will usually work OK running below half power.
You will not likely notice unstable voltage variations, ripple and noise as the motherboard will suppress most of it.
Put a video card in that stresses a cheap PSU and all kinds of gremlins can appear.
So if you upgrade, play a game that pushes the new card and the system gets weird. It's the PSU. Get rid of it...