If you are looking to invest in a new higher end powerful PC you have three main options. Buy a preselected model from a retailer such as PC World, buy a custom PC from a company like Doghouse or build your own. Building your own is far less popular than it once was but remains the best way to get a PC customised to your exact needs at a uninflated price.
The problem with building your own system, apart from the actual assembly, is knowing which parts fit and work together and which parts simply will not. After that its finding the best prices and slotting it all together. The site pcpartpicker.com aims to fix these problems. Pick a CPU and it will offer you every part that will work with that CPU. As you pick each part the list of available parts that are compatible decreases and you can only pick parts that will work together. The site then looks at a range of online sites and compares prices including shipping and any bulk discounts and gives you a link to a basket on each site containing all your parts. It makes a complicated process relatively simple and crucially has a UK customised version with UK retailers.
The site also has an active forum filled with people discussing parts and their relative merits and an area for people to list their builds and experiences. If you are considering building a new PC or even just getting one PCpartpicker.com should be on your list of visited sites.