Alright, I'll amend again! 1680x1050 shouldn't be as much as a strain as 1920x1080, so you'll be alright.
I have amended my second spec above, I changed the corsair PSU for my original seasonic and downgraded the heatsink. The PSU is probably the best on the market as seasonic are amazing, and 550w will carry you perfectly fine. The heatsink is super quiet so won't be loud and intrusive. A 7850 will be perfect at that resolution and even at 1080p, Samsung RAM is on offer this week and is a steal at that price for the performance is brings to the table. Case is kitted out with USB 3.0, and everything else is good quality and what I would get with that budget. CPU is OEM becuase there's no point paying £10 for the heatsink you won't be using, granted you lose the 3 years warranty to 1 year, but CPU's very rarely fail if ever.
Hope this helps.