Reputation: 10I personally would recommend you build it yourself. It's not very hard and will really only takes up an afternoon
Athlon II X4 (OEM + cheap cooler instead of the retail box)
2x 2gb DDR3
NVidia Geforce GTS450
mATX AM3 or AM3+ board (MSI/Gigabyte/Asus; go for the cheapest one with decent ratings)
Cheap hard drive (320~1T
Any CD/DVD burner of your choice
400~500W power supply (Antec, Cooler Master, XFX, corsair, OCZ)
A mid-tower case to put everything in (design is your choice obvs, just don't go for ones with bad ratings)
Windows 7 64-bit Home Premium (get it OEM; retail is like $170 while OEM is $100)
And that should fit in your $600 price range quite easily. Get an AM3+ motherboard if you want to upgrade in the near future, and something like the HD6870 or GTX560 would be a stronger graphics card for the more intensive stuff today. just look up this stuff on Newegg and you should be fine. Sorry I'm not linking anything since I'm on the job atm :x