you also need to sit down and think about what level of PC gaming you want to play at.
Do you want bare bones entry level (still going to be higher end than last gen consoles)
mid range (which will be on a par with current gen consoles - truth is probably better!)
or a proper gaming rig (which will be superior to any console)
and are you happy with a single screen basic set up or do you want a custom multi screen rig? (in which case, if they are high res you may need to consider a very high end rig with multiple graphics cards in sli/crossfire).
Now I am very intel/nvidia centric so bear in mind there is also AMD but, me personally for anything other than super high end I would be looking at a haswell i5 chip. If you reallly want the best you could go I7, but for me, that money is better saved and put into a better gpu.
You really want a solid state primary drive for your OS. they really do make a big difference (and then a harddisk for installing apps/games too).
RAM wise there is no reason to get less than 8gb, indeed if you are going for even a remotely enthusiast gaming rig I personally would be considering 16gb now, simply because it does not cost that much. 8gb is ok for now tho.
and finally gpu..... Which is essentially the big thing for what will most likely be your bottleneck. Low end, if you just want to be able to play most games, and are not bothered about eye candy, a GTX750ti is not a bad choice... you may even get away with one of the AMD APU solutions which has the gpu and cpu built in to 1 chip which will save you a lot of cash.
for middle of the road a GTX770 is a good choice, A GTX760 is "OK" I am running a card about equivalent to that now, and many games I can close to max out at 1080P, but I cant do that on some games, and things are only going to get worse going forward.
for high end a GTX780 is a hell of a nice card. A pricing error from scan 6 months ago, a mate picked up a zotac GTX 780 overclocked edition for £400 (that was cheap then). It is a monster of a card., and if you are feeling really flush for an extra £100 or so on top of the GTX780, you can go up again to the GTX780ti.
If you are getting a ready made rig, you wont have to worry about the psu etc, but that being said, I personally would make sure it had a 600W + one in there and was of reputable quality. There is nothing like coming to upgrade your machine in the future and having to spend an extra £70 for a new psu.
And that is where I stop as I have never built / used an ultra high end rig. Such a rig would have sli GTX780's in there or perhaps a titan black or 2, an I7 processor, and things such as the onboard sound would be disabled and proper stand alone cards used, but that is way beyond my wallet.
overclockers will build you a machine pretty much to any spec your wallet can take, if you really dont want to build it yourself
PS cheaters are the bain of PC gaming, and the downside of using an open platform to game. It is the 1 area where it is hard to argue that the consoles generally have a massive edge.