XFX is good. They offer a lifetime warranty on their cards which is why they are a bit overpriced. I've heard horror stories about warranty.
Asus does not compromise a thing on quality. Their cards are probably at the top of the list when it comes to quality. They offer a 5 year warranty I believe on the video cards which is superior and their warranty service is great as well.
Personally I'd go Asus before XFX for a video card. Otherwise they are the same video card, same amount of ram, the difference being mainly the heatsink/fan cooling design used. Different manufacturers use their own design for cooling video cards. I always choose Asus or Gigabyte for video cards. I stay clear of sapphire, power color, etc... which seem to fail early.
ASUS one has DCUII, XFX one seems to have regular/reference cooler. I think the main difference of these two would be the warranty and the cooler as stated above, because they are obviously the same card. XFX one has dual lifetime warranty, but ASUS has got only 3 years.
I would personally go for ASUS in this because of the cooler, and warranty doesn't matter to me as in 3 years time I would probably do an upgrade.
If you want more real 1920x1080 performance, you can get this 6850 (also an ASUS DCUII card) which is on sale on Overclockers uk. It costs 20 GBP more.
No 6850 is pretty efficient and only need one 6-pin PCI-E power connection to run (just like the 6770). So it's not that 'costly' and 'uses a lot of power'. Your CM Silent Pro M700 Should be more than enough for a 6850.
Your Setup looks fine with 6850
We are not sure about 'Will I be able to run all games on high?', but you can look on benchmark how the 6850 performs. There is one on tom's hardware too.
I can say 6850 is worth it for the 20 GBP you spend. It delivers good performance over 6770.