Not sure where to Start, upgrading PC

Hey guys, I'm not normally a regular poster in the threads here but I do come here for research with questions I may have with any small problems or tips or tricks with hardware/software knowledge building. Thought I'd start here before furthering my dive into upgrading my system that I built in 2011, I haven't been keeping up to date with hardware since that day when I was finished upgrading. I don't think I had planned on building a system that could be upgraded, which is possibly the problem. If i can get some idea's where to start first. that would be very helpful.

I am a gamer, and I do a lot of Audio work/ Podcasts/ Music/ and Audio drama's also.




My sytem-


Graphics Card AMD Radeon HD 6900
System Model GA-890GPA-UD3H
System Type x64-based PC
Processor AMD Phenom™ II X6 1090T Processor, 3200 Mhz, 6 Core(s), 6 Logical Processor(s)
BIOS Version/Date Award Software International, Inc. FF, 11/24/2010
SMBIOS Version 2.4
Installed Physical Memory (RAM) 16.0 GB
2 1 SATA Terabyte hard drives
Thermaltake TR2 TR-600 600w ATX12v v2.3 SLl ReadyFire Ready Power supply.





Like I said, I'm going to go and try to up grade from what I got, but I'm not really sure where I should start, and my budget is sitting at about 700 right now, and it'll be a gradual build, so I'm in no hurry.



Thanks guys for your suggestions!