I have an old extra rig with 4GB DDR2-800, XFX G31 Mainboard, XFX 7600GT and Core 2 Duo E6850 @ 3.0Ghz.
I was considering an upgrade on the GPU to a Gigabyte GTX760 OC (GV-N760OC-2GD) which in my head would make for a good secondary gaming unit - Will this work or will I experience any kinds of bottlenecks?
No offence but the CPU is almost 7 years old.
For a secondary gaming machine I'd get GTX 750TI, and stick to medium resolution gaming.
If you prefer max performance for money (I prefer latest tech) you can also get R7 260x or GTX 660 max.
Spending more money will have little effect on the performance.