Hi this will be my first build ever and it is for my brother for his birthday. I want to build him a gaming computer. He mainly plays WoW and other MMOs. I am trying to stay under $600 and I have already put this together and just wanted other peoples opinion on it before I make the purchase. Thanks. I am also debating on two motherboards. The GPU I will temporarily be using is a GTX 550ti that I used to have in my dell until I have the money to upgrade that.
Rosewill CHALLENGER-U3 Black Gaming ATX Mid Tower Computer Case
GIGABYTE GA-Z77-D3H LGA 1155 Intel Z77 HDMI SATA 6Gb/s USB 3.0 ATX Intel Motherboard or
ASRock Z68 PRO3 GEN3
OCZ ModXStream Pro 600W Modular High Performance Power Supply compatible with Intel Sandybridge Core i3 i5 i7
+1 to AMD Radeon's build. You can easily get the Graphics card in for around $600. The i5-2500K is only necessary if you plan to OC and its one generation older anyway. Since your apparently not OC'ing, you can use an H77 chipset for a lot less money. Many of your other components are overpriced as well. You could even go to a i5-3350P to save a few more dollars without sacrificing significantly in performance. Case is your choice but I would suggest keeping the cost of the case down around $50 on a $600 build.
The obvious advantage of ordering form multiple sites is that you can wind up with a lower total bill. However, if you want to order all from one site, that's up to you. It won't affect the quality of the parts. However, it might be easier to process returns and help with getting everything all at once if you order everthing from one site.