Maximum PC

Hello, I'm new to the forums and I need help finding parts for a budget gaming PC. I tried to build a gaming PC back in 2009 but real life and college got in the way and I only ended up getting the case and graphics card before I stopped so I need help finding the remaining parts and components for it.

Here is what I have:
Case-Cooler Master Cosmos 1000
Graphics Card- Sapphire ATI Radeon HD 5850 1GB GDDR5 CrossFireX Ready

My budget right now is $600-$800 right now. For the monitor I plan on using 1 I already have from a previous PC. I plan to use to play games like Diablo 3 and Guild Wars 2 in high settings if possible.
Any help would be appreciate it. Thank you

Nice chassis - here's a decent build completion just off the top...

PCPartPicker part list / Price breakdown by merchant / Benchmarks

CPU: Intel Core i5-2500K 3.3GHz Quad-Core Processor ($179.99 @ Microcenter)
CPU Cooler: Cooler Master Hyper 212 Plus 76.8 CFM Sleeve Bearing CPU Cooler ($19.99 @ Newegg)
Motherboard: Biostar TZ77B ATX LGA1155 Motherboard ($79.99 @ Newegg)
Memory: G.Skill Ripjaws X Series 16GB (2 x 8GB) DDR3-1600 Memory ($114.99 @ Newegg)
Hard Drive: Samsung Spinpoint F3 1TB 3.5" 7200RPM Internal Hard Drive ($89.99 @ Newegg)
Power Supply: PC Power & Cooling 600W ATX12V Power Supply ($66.44 @ NCIX US)
Optical Drive: LG GH22NS90B DVD/CD Writer ($16.99 @ Newegg)
Operating System: Microsoft Windows 7 Home Premium SP1 (64-bit) ($99.99 @ Newegg)
Keyboard: Rosewill RK-800G Wired Gaming Keyboard ($11.99 @ Newegg)
Mouse: Cyborg CCB437030002/04/1 Wired Laser Mouse ($44.97 @ Newegg)

Total (before mail-in rebates): $785.33
Mail-in Rebates: $-40.00
Total: $745.33

(Using your selected merchants and only including nearby in-store pickup prices)
Lowest Possible Total: $725.33

(Prices include shipping and discounts when available.)
(Generated by PCPartPicker 2012-04-29 10:22 EDT-0400)

If you are nearby a microcenter brick-and-mortar store see if they have the 2500k/z77 combo it can save another $40 (a really killer deal), wasn't sure about the OS, keyboard and mouse so I threw them in (the mouse is very cool)

hope it gives you some ideas (along with others' comments as well)

cheers

Awesome, thanks for the advice. I shouldn't have any trouble with the motherboard and the video card since the video card is a couple years older, right?