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Check out Anand's awesome buyer's guides.
They made a new guide just last week:
http://www.anandtech.com/show/6229/midrange-system-buyers-guide/2
My advice:
Best gaming CPU for your money: i5-3570K.
Best motherboard: Z77 chipset. Pick one of the major brands: Asus, Gigabyte, MSI or ASRock. I like ASRock, because they are still a bit cheaper, while the quality is on the same level as the other brands (aka not overpriced).
Best videocard: I only know the nVidia cards. I'd say a gtx660ti is the best bang for the buck. If you have a little more cash, get a gtx670. If you have enough cash, maybe even a gtx680. After all, putting money in the videocard will get you the most frames/sec per dollar.
AMD has similar cards. In the 7000 series. Others might be able to tell you which ones fit with what budget. I prefer nVidia myself (it seems they work a bit closer with game companies, and thus nVidia runs a bit smoother at launch. AMD usually catches up within 1-2 months after launch).
8 GB of RAM is enough. 4 GB is probably enough for today's games. But RAM is so cheap, it's not worth it to get only 4 GB. Don't get a noname brand, but for the rest it hardly matters what you get. Just get DDR-1600/PC3-12800 (or something faster, if the price is the same).
If you can afford it, get a SSD. For your OS, plus maybe a few games. 60GB can be enough, but if you switch games often, you'll have to copy/move data around a lot. 128GB is maybe the sweetspot these days.
PSU, case, HDD, all matter less for a gaming machine.
Get decent stuff, but if you have any money left, better invest it in a nice monitor. (I'd go for a 2ms 1920x1080 27", if I were you and the budget would allow it).
Hope this helps.