As I see it you have two options, so I provided two builds. One that will last you that you can upgrade over time, and one that is good now and will be for a few years.
Futureproof:CPU: Intel Core i5-3570K (http://www.newegg.com/Product/Product.aspx?Item=N82E16819116504) $220
Motherboard: ASRock Z77 Extreme 4 (http://www.newegg.com/Product/Product.aspx?Item=N82E16813157293) $120 (includes 8GB RAM you can sell)
PSU: Corsair TX650w (http://www.newegg.com/Product/Product.aspx?Item=N82E16817139005) $80 (I have this PSU and I have not had any problems)
GPU: Scrap (for now)
HDD: Scrap (for now)
Case: HAF 912 (http://www.newegg.com/Product/Product.aspx?Item=N82E16811119233) $60
RAM: G.SKILL Ripjaws 2 x 8GB 1600 (http://www.newegg.com/Product/Product.aspx?Item=N82E16820231568) $130
Windows 7 64-bit (http://www.newegg.com/Product/Product.aspx?Item=N82E16832116986) $100 (Or, you could find an alternative method...)
Asus Xonar DG (http://www.newegg.com/Product/Product.aspx?Item=N82E16829132020&Tpk=asus%20xonar%20dg&IsVirtualParent=1) $30 (As a music enthusiast I love this card. Great for mixing/composing.)
Comes in just a bit overbudget after tax/shipping.
Good now: CPU: AMD FX-6300 6-core @ 3.5GHz (http://www.newegg.com/Product/Product.aspx?Item=N82E16819113286) $130
Motherboard:: ASRock 970 EXTREME4 AM3+ (http://www.newegg.com/Product/Product.aspx?Item=N82E16813157262) $100 (I have something similar to this.)
PSU: Rosewill Green Series 630w (http://www.newegg.com/Product/Product.aspx?Item=N82E16817182200) $60
GPU: Sapphire 2GB 7870 (http://www.newegg.com/Product/Product.aspx?Item=N82E16814202025) $210 (Includes 4 games. Pick it up.)
HDD: Scrap (For now)
Case: Rosewill Challenger (http://www.newegg.com/Product/Product.aspx?Item=N82E16811147153) $50
RAM: G.SKILL Ripjaws 2 x 4GB 1600 (http://www.newegg.com/Product/Product.aspx?Item=N82E16820231314) $60(You can add two more sticks later.)
Windows 7 64-bit (http://www.newegg.com/Product/Product.aspx?Item=N82E16832116986) $100
This one is a bit closer to budget but I had to cut the sound card to accommodate a nicer GPU.