I don't know how trustworthy "BEDIR SHOP" is, you might have better luck with the international sister company of where I got my computer from:
http://www.amazon.co.uk/gp/aag/main/ref ... HKR69F1Q48
Note that I had mine custom built, if they only offer pre-built computers in the UK you might want to check that it has a decent power supply from Corsair or something.
ALSO if you end up getting an XFX GPU or anything like that, always check to see if the parts require you to register on the manufacturer's website to obtain a warranty. XFX has a "double lifetime" warranty that you can only get if you register within a few weeks of obtaining it. I already knew about this, but some people don't. I've heard that they replace older models of XFX GPUs that fail with a newer model for free though, just make sure the newer one is compatible before shoving it in.
Knowing how to build/maintain a PC is good even if you go with a pre-built PC, can help you troubleshoot issues and other problems in the future, and you learn how to upgrade your GPU or RAM in the future.
NewEgg and
Tiger Direct have good videos on YouTube with instructions on building/maintaining PCs
EDIT: oh right, price.
Go with as expensive as comfortable for you, don't "hold back" to "save money", if you simply can't afford something then obviously you won't be buying that.
You could just save some rainy day pc cash on the side, but for the most part you want to get a pc that's got enough power to last you some years.