Case: The CM HAF X is easily one of my favorite cases for cooling potential and looks (the blue LED version atleast). However, I don't think I could personally justify spending $180-$200 on a case, especially on a budget. I really like the recently released Corsair Carbide 400R
and the 500R that is coming 9/23 ($100-$140).
Processor: 6-Core processors won't have any benefit to gamers since no games out (or coming soon) utilize more that 4 cores. The Intel i5 2500k is probably the best gaming CPU available right now. The stock HS/F for the i5 is fine for stock speeds, but you certainly want an aftermarket if you plan on overclocking it. Depending on how high you would overclock aftermarket HS/Fs can run $30-$100+.
Motherboard: For the i5 2500k you need a socket 1155 motherboard and either a P67 or Z68 northbridge. I'm personally a fan of P67, and its arguably the most popular choice.
RAM: You would have to upgrade to dual channel DDR3 for the new motherboard.
Graphics: GTX 580, awesome.
Power supply: I am curious as to the manufacturer. An unreliable PSU can fry your other components, but if its been working fine for you for this long maybe it won't blow up your new parts...
Disc Drive: Generally $20 for your generic 24x DVD/CD burner.
Quick and Dirty:
CPU + MB: $350 i5 2500k + Asus P8P67
RAM: $33 4GB G.Skill Ripjaws X 1600 CAS9
GPU: $500 MSI GTX 580 Twin Frozr II/OC
ODD: $20 Sony Optiarc 24x DVD/CD Burner
Already hit $900 and thats before a Case, monitor, CPU cooler, and Windows 7.
If you go AMD you can get CPU, MB, Win7 for $327 and use the other parts; but you'll be at $880 w/o a case, monitor, cpu cooler.
CPU + OS: $217 AMD Phenom II x4 965 BE + Win7
MB: $110 ASRock 970 Extreme4 AM3+
Might consider dropping down to a GTX 570, same MSI Twin Frozr II OC
($20 rebate) model will save you about $145 to put toward other parts.