I've built a couple of gaming PCs. It's surprisingly simple, so long as you're fairly confident/competent with a screwdriver, other than a bit of patience and a little bravery it really doesn't require much more of you. For the process of actually building one, I bought my self a 'How to build a PC' magazine about 7/8 years ago and it was still a good guide when it came to building my current PC about 3 months ago. So get yourself one of those magazines, the info might be available for free elsewhere but having a hard copy to hand for reference is very reassuring.
As for choosing components, take your time, spend at least a month going to various websites asking for details and opinions. It's like tying together a dozen strands and can seem overwhelming when you first start but it's worth the effort. Do your research. It's easy to get lost in all sorts of details and online opinions, so take some time to get a couple of viewpoints on something.
A good rule of thumb is to pick the best processor in your price range. The big differences between processors come with each new generation, not within a generation so just get yourself the best you can afford, cause you're likely to be sticking with it. NB unless you're doing heavy graphics design work don't bother with a quad core, a dual core is plenty for gaming and it'll save you a bit of cash.
Then you need to pick a compatible motherboard, and a case they'll fit in. With two main camps, Intel and AMD processors they are different and have their own compatible motherboards (eg an intel chip won't fit an AMD board) and it's also possible to overspend on a motherboard, a mid range one will see you right. With respect to cases, a lot of this is standardized, so it's mainly down to personal taste.
Graphics cards, as you're thinking of bulding a gaming PC you're gonna want to spend a good chunk of your money on this. Beware, there are fanboys out there when it comes to graphics so go to respectable review sites to find out what's right for you. I'd say a ratio of about 2:1 on price compared to your processor will give you a really good starting point (so eg £100 processor = £200 graphics card) however, do remember that this component is relatively easy to replace/upgrade (if a little expensive) so you might want to consider that when making your list.
RAM is one of the cheapest components, do a little reseach into 1333 and 1666 (bus speeds I believe?) but generally the higher the number the better, and again, very much so with RAM, it's usually just a matter of sticking £20 worth in extra at a later date. 4gb is a good start and 8 gb is honestly plenty, a very easy upgrade at a later date.
Storage. 1tb, fairly cheap brand. Standard hard drives are reliable and fairly cheap nowadays.
Power unit. PSU is kinda important, once you've made the descisions on the other components you should add up their power requirements and (baring in mind future upgrades) choose the appropriate unit for you.
Operating system. Don't forget to leave room in your budget for windows (if you want it, there are other, free options but i don't have experience with those so won't comment). Although it might seem like a bit of a hidden cost, having your own copy does have it's benefits, mainly allowing you to do a complete system wipe and install. This gets your PC back working the way it was as it came out of the box. Get the 64 bit versions if you're going 4gb or more of ram, 32 bits will only recognise 3gb of ram.
Optional extras you might want to consider are extra cooling for your processor, although Intel (not sure about AMD) processors come with a stock fan that's fine for a first build. Extra cooling for your case, check your motherboard and how many fans it can power, and your case and how many fans it has room for. These cooling options can be done fairly cheap at a later date. With storage you might want to consider an SSD drive, about 4 times more expensive, all it does is make you frustrated with how slow other computers are in comparison, an epic addition (from the year 3013), though choose carefully as some have reliability issues. Thermal paste for your processor is pretty much a must, it just helps keep it cool.
By no means a complete guide, these are just some pointers. Most importantly, take your time when considering components, ask for advice and get that advice from multiple sources when possible. First port of call is the 'How to build a PC' magazine.
EDIT: Oh yeah, optical drive. DVD writer will do
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Last edited by King Gary; 02-13-13 at 02:43 PM.