The minimum for the HD6850 would be 400W, while the recommended is 450W. .

It's not recommended to stick to the minimum recommendation. Get a power supply like the FSP Hexa 500W.

Something with ~350W on the 12V rail(s) would be more than enough for your needs.How much are you willing to spend?

Gigabyte Odin everywhere also 300++. Cant find rm169 one.

Well,you can get the Seasonic X560 for RM350~,that's a rock solid PSU but imo it isn't worth the price.The Odin GT is a good PSU but it's rather hard to find these days at that price (RM169].With that said i recommend the Corsair CX500 that is usually sold RM220,it has 400W of power on the 12V rail and it's based on a proven platform.

Although the Seasonic X-560 is an excellent power supply ( the X-series of Seasonic is very good according to reviews around the net ), it's priced at RM359, which is rather expensive. You have some other alternatives out there e.g the Silverstone Strider Essential 600W which is cheaper, albeit not as good as the Seasonic X-560 in quality. In addition to that, if your budget is at RM359, then you have the option of getting the Seasonic Silence Pro 750W for RM329 from the Garage Sales section. It ( 750W ) has the ability to output more power than the Seasonic X-560 and it costs cheaper, although it's less efficient and it's not modular. In other words, the Seasonic X-series power supplies are premium power supplies and you should only go for them if you really have the money to spend and you would like to get yourself the best of everything.

Not all modular power supplies are expensive, or perhaps your definition of expensive is different .

Anyway, yes, technically a power supply with the ability to output e.g 800W that runs a system consuming only around 400W, which is 50% of its capabilities should have a higher efficiency than a power supply with 450W of power running a 400W of power. Most of the power supplies runs has the best efficiency at half their load and reduced efficiency at higher or lower loads according to most of the power supply efficiency graph I have seen before, for example these two :

Well, actually a better choice than the available power supplies in that range would be the Gigabyte Odin GT 550W that costs RM169 from Jayacom. Unfortunately, its availability may be a problem, and if it's really unavailable, then you can choose to go with the Silverstone Strider Essential 600W for RM279 from Viewnet, or the Seasonic Silence Pro 750W for RM329 ( without postage cost ) if you're able to spend more than RM300.

Xigmatek 750W? I thought that is quite old already? my previous PSU

new one is 700W. And the answer is no, why?
Seasonic 750W which is much better, cost cheaper and IINM, the warranty is longer.

IINM = If I'm Not Mistaken

Only if you're planning to buy, then get the Seasonic one. But since you have one lying at your bro's home, then just use that.

wait, you're planning to get one HD6850 only right?
then something cheap like FSP Hexa 500W will do.