Desktop PC, all the way, especially with all the cool game streaming stuff on the way. e.g. from a Nvidia Shield you can play games running on your desktop, and a cheapish steam machine will allow you to do a similar thing with the TV in your lounge.
The upgradability of a desktop is also king. I regret buying a laptop when I turned 18, as three years it got thrown out because one thing broke and it had gotten really slow.
As for what to look for:
There was a time a short while ago that you would be laughed at (by me
) if you went with an AMD cpu. It's not so bad now, but Intel still trumps AMD for gaming when you have a dedicated graphics card.
AMD do do very good "APUs" which are CPUs with on board Radeon graphics. These can do really well in low end gaming PCs and benefit a lot from faster memory (otherwise memory speed doesn't really matter.) But I'll assume you're going a little higher end and are going to want a dedicated graphics card.
Get an Intel "Haswell" i5 (say the i5-4670k, or the non-k varient if you don't want to tinker with overclocking), it's a fast quad core and overkill for gaming.
If you have money to burn, you could bump that up to an i7 (i7-4770k), it's a quad core as well, but with hyperthreading, so that Windows will see 8 cpus instead of 4. You wont really seen any benefit in gaming, but could do if you do video editing, etc (hell, I still run a dual core and am only just considering an upgrade.)
This comes down to a choice between AMD and Nvidia. Traditionally I have been on the AMD side of the fence (I currently have an AMD Radeon HD 6950), but to my eye, Nvidia is the better choice of the current generation. If I were to buy new today, it'd be trying to save for a 780 ti.)
Performance of the card itself isn't the only factor through. Some games support one manufacturer better than the other in some cases. It may pay to look up benchmarks on the specific games you're thinking of playing most.
Games tend to come bundled with cards, so this is something to consider.
Features also differ. Nvidia has the Shield streaming I mentioned earlier, shadow play for doing a lot of the work of streaming your gameplay on the card so it is easier on your system, and g-sync which is coming to monitors and is really compelling to me (this is a whole nother topic... check out this video if you're interested: http://www.youtube.com/watch?v=3PJjhBUSuHk
). AMD has Mantel, which is an API that is meant to be a lot better for gaming performance than Direct X. Both g-sync and Mantel may be cross platform at some stage and blah blah, but at this stage it's better to go for the card that you know will support what you want.)
I can't tell you what is best, only what I would get. Honestly, as it's the best current performance, I would try above all else to get the 780 ti.
Stick to one of the big names and you should be good, I'm thinking of ASUS, Gigabyte, MSI. You're looking for the Z87 chipset. Everything this generation is much of a muchness. As you go up the price brackets, you're just loading up with more features (thunderbolt, special network ports, overlocking niceties)
I shop by color
I try and adhere to a colour scheme so might choose a board based on the fact that it is red and black, or just black, or has a little white on it.
Other things to look for
Memory speed doesn't matter too much, get something that looks nice, is 1600 MHZ or more.
Haswell chips and the latest graphics cards are reasonably power friendly so you don't need a giant 1,000W beast. You should be good with something around the 650W mark, maybe look about 800W If you're thinking of going with two GPUs. Also the 80 PLUS ratings you'll see have become something of a marketing race... 80 Plus Bronze should be fine, but going for 80 Plus Gold won't hurt anything.
Solid State drives are so much faster than HDDs that just getting one of these can make an old system feel new again without any other upgrades. My preferred storage config these days is to get a SSD large enough for Windows, a few apps and your most played games (or even better, an SSD for windows and a separate one for games and apps) then a giant (3 or 4 TB mechanical HDD) for everything else.
Case, I prefer Corsair cases, also like Coolermaster and Fractal. Shop by looks
Everything else wont really make or break a gaming PC. If you want to open up a can of worms, consider looking into PC audio, soundcards, DACs and AMPs, etc. Also don't get me started on keyboards
Where'd a pluck all this stuff from?
I work in IT and also watch many hours of tech videos daily
. I try and keep up on all the new releases, etc. Have built a few custom PCs for people around here in the last year and my work also got me to build 5 supped up "workstations" (fully of gaming equipment) for our GIS/Mapping department.
I also need a life
In saying that, if you need help deciding on something, or have a parts list or a couple of options for prebuilt machines, feel free to ask. I'm always happy to help out with this sort of stuff.