i realy have to agree with tom here. upgrading your cpu or gpu would be a waist of money imo.....you will gain only a little bit of performance for your money. if you like quiet, maybe upgrade your case to a quiet edition case like a fractal R4, corsair 550D, or something along those lines. also maybe a better, quieter cooler such as a H100i, or a NH-D14? i dunno lol....your system seems kinda solid! but i did build a system for my dad useing a fractal 3000, and i did find it a little loud with the stock fans. so if you like quiet i just recomend a case.
2500K@4.4, CM Hyper 212+, 16GB Kingston HyperX 1600, Gigabyte Z68X UD3, Gigabyte 760 windforce OC, Corsair AX760i, Corsair 550D, 256GB OCZ Revodrive 3, 1.5 TB WD Black, 27" LG IPS, B&W 601 S3, Yamaha RX-V420