The issue is that you don't need anywhere near that much power. A good quality 400W PSU is more than suffcient for that build...even if you got and HD6850 you wouldn't need mre than a good 450W PSU. If you want some extra power for future upgrades, then you could get the XFX Core 550W for 50 quid and free shipping from Amazon: http://www.amazon.co.uk/gp/product/B004RJ8EKI/ref=s9_si...
Review: http://www.jonnyguru.com/modules.php?name=NDReviews&op=... You don't need anything more than that unless you're going to SLI/CF some mid-high powered GPUs....and that XFX 550W will handle a CF of HD 6850s
^+1, the E3 series of the GX models failed every review due to excessive ripple on the 3.3 V rail.
One change i would make to your build, the most important thing in how responsive your system feels is the speed of the main drive, that 5400 RPM drive is going to feel and be real slow compared to a good 7200 RPM drive, i would drop down to 8GB of RAM and spend the extra 36 on getting a good fast hard drive, since a laptop hard drive like that scorpio is going to be real slow.
There really isn't a disadvantage other than upfront costs. There is only a disadvantage if you get a massively oversized PSU for your system, if you build a micro ATX system with onboard graphics and use a 1kW PSU then you start to hit the point where the fact that efficiency is low at very low load % starts to matter, but for the most part a normal system has no advantages or disadvantages with picking a PSU between 550-850 other than the upfront cost.
Also do you need a 120HZ monitor? As far as gaming goes you are never going to get close to rendering 120FPS even if you get a slightly better GPU so buying a 120Hz monitor seems wasteful better get a normal 60Hz and splash more cash on the actual machine.
Normally I would go in to a list of reasons not to buy FX but today I cant be bothered and with the cashback offer its actually pretty cheap/