Tom's Hardware is a good place to look. They've got monthly updates on the "best buy" within different price ranges when it comes to CPU and GPU. I always use that as a starting point of my build.
I wouldn't recommend anything but an i5 or maybe i7. Intel processors are not that much higher priced than AMD ones, and in return you get higher performance, lower power consumption, lower heat development and generally just more "advanced" technology (this is coming from a former PowerPC fanboy, so I'm not really biased towards Intel, they just ARE ahead at the moment and have been for quite a while) - you also get stuff like PCI Express 3.0 (the slot you chuck your graphic card into) which has higher bandwidth than what you get on a AMD board. i7 is obviously "better" than i5, but for gaming it is questionable if it is worth the extra $ - that also depends on what your budget is, the link to Tom's Hardware above will give you a good idea what to get within your budget.
When you know what CPU you want, you can choose a motherboard/chipset that fits it; i.e. if you go for a 4th generation i5 (the ones labelled i5-4xxx), get a motherboard with a 4th generation chipset. They always carry marketing names like "Ivy Bridge" or "Haswell", so it's not that difficult to find out what matches. Also this is an area where you can save a bit of dough if you go for a mobo with just 1 PCI Express slot and a matching chipset. You wouldn't want to stick two graphic cards into it anyways, not worth the trouble - far better to just throw out the old one and buy a new one when you need to upgrade.
Then RAM, I'd advise you just get the cheapest DDR3 ram (I think it's 1333 Mhz) from a respectable producer. And get heaps of it, prices are only going to go up from here, making a future RAM upgrade far more costly than it has to be.
I would also get a SSD if possible, I have a 128gb one for my OS, the majority of the application I use and a few games. It's nice because you get super fast startup and you recover from sleep instantly, in general improving user experience by a whole lot. Then get a cheap 1tb or similar old fashion HDD for storage.
GPU wise I have an AMD but I tend to advise you to get a Nvidia. It is a bit tricky and full of conjecture, but at the moment Nvidia offer better drivers and better support in games; however the two new consoles are built with AMD hardware, so most games in the future will be directly developed for AMD GPU's, then ported to PC's. It might be an advantage to AMD PC users, it might not. As it is now, roughly put, you get more bang for the buck going the AMD route, but higher quality and better performance and driver support using Nvidia. Use Tom's Hardware and keep an eye out for your local prices, if they have a super good offer on either one just go ahead and grab that (when I made my build I was gunning for a Nvidia 650 but found a AMD 7950 at the same price so it was a no-brainer to go with the AMD GPU).
