Seeking upgrade advice, less than full build.

Hello BuildAPCForMe! I hope this is the right place to ask this question, as it isn't a full build I am looking for, but rather areas I can improve my current machine. I'll fill out the questionnaire and throw in my current specs further down:

What will you be doing with this PC? Be as specific as possible, and include specific games or programs you will be using.

This is my primary home machine that I use to surf the net and play games on. It's hard to be specific with regards to games, since I try to play all of the major AAA releases as they come out. Looking forward to Witcher 3 on May 19th! I'm a casual gamer and I'm not fussy about being on the absolute bleeding edge, but I do enjoy playing games on the higher settings at the native resolution of my monitor (1920x1080).

What is your maximum budget before rebates/shipping/taxes?

Not really applicable. Looking for best bang for buck, and don't really have a set ceiling for budget. I would like recommendations to be based on actual performance increases, if those gains are worth spending money to achieve.

When do you plan on building/buying the PC? Note: beyond a week or two from today means any build you receive will be out of date when you want to buy.

Immediate, if suggested.

What, exactly, do you need included in the budget? (Tower/OS/monitor/keyboard/mouse/etc)

This is where the unique nature of my request swims against the current. There is nothing that I need, as I have a machine that is doing its job reasonably well.

Which country (and state/province) will you be purchasing the parts in? If you're in US, do you have access to a Microcenter location?

Ontario, Canada. I have a preference for NewEgg.ca but will follow the deals, provided shipping doesn't outweigh savings (of course).

If reusing any parts (including monitor(s)/keyboard/mouse/etc), what parts will you be reusing? Brands and models are appreciated.

Anything that isn't upgraded. :)

Will you be overclocking? If yes, are you interested in overclocking right away, or down the line? CPU and/or GPU?

I have never overclocked anything.

Are there any specific features or items you want/need in the build? (ex: SSD, large amount of storage or a RAID setup, CUDA or OpenCL support, etc)

No, but feel free to offer suggestions.

Do you have any specific case preferences (Size like ITX/microATX/mid-tower/full-tower, styles, colors, window or not, LED lighting, etc), or a particular color theme preference for the components?

I would like to retain my current case.

Do you need a copy of Windows 7 or 8.1 included in the budget? If you do need one included, do you have a preference for one or the other?

I am currently running a legitimate copy of 8.1.

Extra info or particulars:

Alright, so here's the real meat of the post! My current specs:

Case: Antec "ELEVEN HUNDRED"

Power Supply: Corsair HX750

Mobo: Gigabyte GA-Z77X-UD3H

CPU: Intel i5 3570K @ 3.5GHz

GPU: GTX 770

RAM: 16gb DDR3

Storage: Samsung 840? SSD (128gb), 3x WD 750gb HDD

Monitor: 2x Samsung SyncMaster S27B350 (27" LED)

So, what can I upgrade that will result in practical performance increases that will allow me to extend the useful life of the machine? I know I could upgrade the CPU and GPU, but to which? Which would offer the best balance of cost vs performance?

Thanks!