Hello PC Advisor!

Sorry if this is in the wrong forum, it's my first time posting here :) I've searched for similar threads but the only ones I've found concerning my specific PC are a few years old so the information is outdated.

Anyway, my question is about an Inspiron 570 - I know this isn't specifically a gaming PC but it was a gift and I'd rather upgrade it if possible.

Specs are mostly here, although one or two things I have differ:

Radeon HD 5450

AMD Athlon II x4 635 Processor (4 CPUs), ~2.9GHz

4GB DDR3 RAM

I'd like to get Guild Wars 2 and play it on the highest settings (around 25-30FPS on average is fine) at a 1600x900 resolution, but I don't think the Radeon HD 5450 will even get close to those settings.

So I'm thinking of upgrading my graphics card. I was looking at the Radeon HD 6850 because it doesn't seem quite as good as the Radeon HD 6870 but it's better value, and according to this review it should run the game pretty well.

However, the 300 Watt PSU I have isn't going to power a card that good so I know I'd have to upgrade it too.

I'm on a budget of around £150-175 and these were the two I was looking at:

http://www.ebuyer.com/269259-corsair-600w-cx-series-v2-psu-6x-sata-2x-pci-e-cmpsu-600cxv2uk

http://www.ebuyer.com/391772-xfx-hd-6850-1gb-ddr5-dual-dvi-dual-mini-displayport-hdmi-pci-e-hd-685x-zcfc

So, my question is will these be compatible with the rest of my PC? Will it fit in the case (it's an average sized tower), are there any extra cables I would need to purchase etc.? Is it worth spending the extra £30-50 on the Radeon HD 6870? And lastly is it worth upgrading my ram to 8GB?

I've never really tried to upgrade a PC before, so any advice is very much appreciated!

Thank you in advance! :)