Just bumping it up to 700 or 800 would have a pretty big impact on performance. 600 is a bit low to get you a machine that's up to par with most of todays.
Here's a quick one I built, probably not the best, but still pretty nice.
Mobo - 70
i5 2500k - 220
psu - 60
Gskill 4gb - 30
1TB wd blue -65
Optical - 19
6850 1GB - 160
total $624
You can pick your own case if you like. I'm not very good at it nor do I enjoy picking them out.
Feel free to pick a different motherboard if you need more pci slots or something.
Or a new HD if you want a 2TB or a caviar black. I'd stick with WD though...
just suggestions
WD Blue drives have TERRIBLE reviews tbh. That drive alone has nearly 50% 1 star ratings... Seagate's Barracuda series is really reliable and at a better price point. It's between that or the WD Blacks, imo. Also I'd stay away from Diablotek, not a very reputable PSU maker + it's only 79% efficient. 675W is also waaaay overkill for that system. It'd probably top out at 300W max lol, further increasing the inefficiency. Corsair, Seasonic, Silverstone, Antec, OCZ are all a much better choice IMO. Skimping on a PSU is like fast track to a broken computer.
Other than that your build will work but having a 2500k at a $600 budget seems kind of like having all your eggs in one basket. He'd also need a CPU cooler to even come close to tapping that CPU, which adds even more to the already boosted budget :s. Plus with a case it's gonna tip well over $700 probably... most likely better off with a AMD Phenom II X4 965 unless he can squeeze out the extra money for more budget.
6850 is a good value card but something like
this probably has better price/performance. It's $10 more but performs significantly faster. Same price after rebate too.
@OP
A bigger budget would vastly improve your overall experience. Even if it was just by a couple hundred extra, it would be a big help. Better to save for a longer period of time and buy current tech than have to buy something impulsively that's already a generation or two behind imo.
My recommended build based on your budget:
CD/DVD Burner:
http://www.newegg.com/Product/Product.aspx?Item=N82E16827136236
Antec 300 Case:
http://www.newegg.com/Product/Product.aspx?Item=N82E16811129042
Seagate Barracuda 1TB 7200 RPM HDD:
http://www.newegg.com/Product/Product.aspx?Item=N82E16822148697
GIGABYTE GA-880MA-USB3 AM3+ Motherboard: (supports Bulldozer if you ever wanted to upgrade)
http://www.newegg.com/Product/Product.aspx?Item=N82E16813128490
Sapphire Radeon HD 6870 GPU:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102948
OCZ ModXStream 500W PSU:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817341016
CORSAIR Vengeance 8GB (2x4GB) RAM:
http://www.newegg.com/Product/Product.aspx?Item=N82E16820233186
AMD Phenom II X4 965 CPU:
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103727
Cooler Master Hyper 212+ CPU Cooler:
http://www.newegg.com/Product/Product.aspx?Item=N82E16835103065
Grand Total:* $687.87
Buuuut, there's $65 in MIR + a few items have instant savings (check links for codes) all in all the build will be close if not under the $600 mark when all is said and done and it's everything you need, case etc. That being said, I still say wait and save up more so you can go with a 2500k.