Idk how much you know about gaming pcs so here's a list of things you need to buy:
Graphics card
Processor
Ram
Motherboard
Case
Harddrive
Operating System
Powersupply
Optional - DVD drive
Optional - SSD
Don't skimp on the graphics card it's the most important part, that's the best tip there is for a gaming pc. If you are looking to save money go with AMD I have a 7870 Radeon, it can play bf3 on Ultra, Crysis 3 on High, and Bf4 on high. That card is only ~$180 so that's a personal recommendation from me.
Next the processor is also pretty important, if you're gaming I'd recommend an i5-4670k (I have an i7 but I don't think hyperthreading makes a difference in gaming.) if you're recording too however an 8 core AMD might be better like what BigZ recommended and if you're editing an i7 is the fastest but it only barely beats the amd fx-8350.
Ram isn't too important just make sure to get 8gb or more.
Motherboard just make sure it can accept all the other parts; gpu, cpu, ram. I have a MSI Z87-G45 that's a somewhat fancy one, the motherboard makes no difference on FPS. You only need the extras if you really want them.
The case just needs to be big enough and have some adequate cooling. My recommendation is a Fractal Design R4 it's a really silent case, even when you're maxing out a game or rendering a video you can barely hear a thing.
HDD make sure to get enough space, I have about 50 games installed right now and it's taking up about 500gb space just to put it into perspective.
I have had both W7 and W8, tbh I like W7 a lot more but it's just preference, if you can deal with the ugly metro layout go for it otherwise W7 is gonna be your choice.
The powersupply is more important than you think, I couldn't tell you what psu to buy unless I know all the other parts but don't cheap out on this, if it fails and shorts out your computer you are screwed, try to buy one that is 80 plus certified, prob above 500 watts.
DVD drive is optional because you are able to install windows without the disk, but you would need another computer that does have a disc drive if you wanted to extract windows from the retail disk.
SSDs are completely optional, you can use them to put your operating system on and have your pc boot up in about 6 seconds which is cool af, but they are somewhat expensive ~$100 for 120gb of space.
Best site to buy all this from is Newegg or Amazon.