I've bought quite a few things from Scan, and they have always been spot on. Pretty good with returns and customer service too. As for the build, it depends on what you are planning on doing with it? If it's for gaming, I would recommend uprading the processor, RAM and graphics card if you want to run something top end. The forums over at CustomPC are normally a good place for advice.
That's a good decent all-round machine you have, no bad video card and excellent processor. It's all in the `bang-for-bucks' bracket. It will play the latest games - just not at daft resolutions. The only addition I would say is more RAM, buy 2 x 4GB sticks, it will make a big difference (RAM is VERY cheap at the moment) microdirect.co.uk is a really good site too
pcspecialist.co.uk is top notch too. Great prices and the guys are happy to spend ages talking with you on the phone to suss out your needs. Can't recommend them highly enough