Well, first you want a good price on things, and if you live in the US it's really hard to beat Newegg. Not only are they low, but also very reliable; Newegg has a great return policy too, not that I've ever had to use it, just that it's nice to know it's there. They also supply Canada, but I think their prices for Canada are a tad higher, so you might want to shop around a bit. For those not in the US, I have no idea...sorry, you might have to put a little more effort into the shopping part.
Now, for those that aren't familiar with this, there are two brands really: AMD and Intel. AMD is cheaper but less reliable, Intel is also a little bit better all the way around too, especially if you're wanting a gaming system you'll want Intel and Nvidia. Now I'm probably gonna' get flamed by the AMD lovers...they're just sore because they can't afford the higher end Intel stuff. :p
Get what you want though...AMD isn't bad, just not as good.
So what you'll need is:
1. A case (preferably one with a few fans, I like Antec, they look awesome).
2. Power supply (you want this to be at least a 720 watt, but I have a 1000W so I have room to grow, go for the 1000, you'll thank me later).
3. Motherboard (this is a tad tricky because you need to make sure the CPU and the MB go together, plus you want something serious too. I like Gigabyte Motherboards a lot, they're very stable from my experience. You want to pay attention to certain things though, like how many PCI-E slots there are, you probably want to make sure there's three. How many memory slots there are and what they can take, I personally have six and you want to make sure it takes DDR3. With Newegg these things won't be hard to figure out and you can always ask me.
4. CPU [processor] (Since I really don't know or care what AMD CPUs are like, I'll just tell you about Intel. There's the "i" series which is what is going now and you at least want something like an i5. However, what really counts here is the GHz and the cores. I have an i7 with something like 3.4 GHz, 4 cores and 8 threads so it can take all the crazy applications and stuff I throw at it, but if you're looking for gaming then I'm not sure if more threads and cores is better or worse...maybe someone else knows.
You do want the 64-bit though, make sure you get the 64-bit.)
5. RAM [memory] (Of course you want DDR3 and you want like 6 or so gigabytes. I think most games do okay with 4-6 but I'm not sure about that since I really don't game on my PC. I have 12 gigs of RAM though myself.)
6. Graphics card (Yeah, just go Nvidia...it's designed for games, okay?
There's a few really good companies that make Nvidia cards, but what you really need to look at is the CUDA cores. You really want like a GTX 470 (not GTS, don't get them confused). One GTX 470 or higher and you'll be singing a little song while you look at life spring forth before your eyes. Now, this is where the three PCI-E slots come in though, you see...you can string three graphics cards together with SLI as long as they're the same card; so if you buy one card, then later on you'll have to by the same one again in order to dual SLI the two (then three at some point), so you want to make sure the first one you buy is a monster to begin with.
Note: CUDA power in graphics cards is somewhat "new" development (nothing's really ever new for long though), and the only time they're really used is when playing games. To my knowledge, Nvidia are the only ones that have it, and you want it, trust me.
7. Hard drive (now there are solid state drives which are really nice, but way too expensive for even my taste, wow. So unless you're crazy, you're just going with a good ol' fashioned disk drive. Why not get the 1T though, that's a 1000 gigabytes of storage space without much of a price difference. However, you need to check that read/write speed and compare that stuff, it won't mean much it you have a T but it can't perform. The brand I recommend is Seagate, I think that's just because I've had a bunch and they haven't let me down.)
8. DVD burner (Why not? They aren't that expensive really. LG is a nice brand, but I really don't pay attention to that much...these things are so cheap in comparison to everything else that you won't actually care.
9. Keyboard and mouse (duh).
10. Speakers (just get the Gigaware, don't fight it, it's $75 I think for the nice ones, you really will thank me later).
11. Copy of Windows (or you can go Linux for free, but please don't steal.)
Now set everything up! Oh...wait...you can't see anything! :crazy:
12. Monitor (Yeah, baby, and you thought the monitor was the computer before, didn't ya'... Now you're going to need to pay close attention to the specs and stuff, I actually have spent an hour or two picking out monitors in the past. For that badass graphics card you bought to shine in all its glory, you'll need full HD 1920:1080p. It's more than that though, you have to pay attention to the colors it can display, it's brightness and contrast and its refresh rate. This can get a little tricky actually, don't just look for the big one. O_o)