Upgrading MB, CPU, memory $400ish

Approximate Purchase Date: Now
Budget Range: around 400
System Usage from Most to Least Important: Gaming
Parts Not Required:
Preferred Website(s) for Parts: Amazon, Newegg
Country: Florida, US
Parts Preferences:
Overclocking: Maybe
SLI or Crossfire: No
Monitor Resolution: 1680x1050
Additional Comments:

I’m upgrading my current system an Athlon Dual Core 2.6 Ghz. I was planning on keeping my case, power supply, video card, drives, and peripherals. The games I’ve been playing run poorly. Diablo 3, Guild Wars 2 (beta). I’d like to be able to run them smoothly on medium/high.

Parts I’d like to keep from my current system:

Case: Antec Three Hundred
http://www.amazon.com/Antec-Three-Hundred-Gaming-Comput...
Power Supply: LSP 550 Watt
http://www.tigerdirect.com/applications/SearchTools/ite...
Graphics Card: GIGABYTE GeForce GTX 560
http://www.amazon.com/GIGABYTE-GeForce-mini-HDMI-Graphi...

Parts I was thinking on buying:

Motherboard: ASUS P8Z68-V/GEN3
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
CPU: i5 2500K
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Memory: Patriot DDR3 8 GB 1600MHz
http://www.amazon.com/Patriot-Division-PC3-12800-8-9-8-...

I haven’t built a computer in years. I’ve never tried overclocking either. I was initially thinking AMD again but everyone has been strongly recommending Intel. I’m also hoping my current power supply will be OK?

Thanks for any advice.