not in the computer hardware trade but been building my own windows computers since 1991

upgrading your computer - not possible to give specific advice without knowing what your motherboard actually is; it is unwise to buy a component and assume it will work with your system, you would need to get advice as Chris said

About 4 years ago I bought my current windows 10 pro system:
windows 10 that's a given unless you wish to run mac/linux and I see no reason to unless you need a specific program that does not run on windows
intel 8 core i7-4790 cpu @ 3.60 ghz
should be fine provided nothing else in the system is throttling it
8 gb ram
a bit low, I would say 16 GB minimum, and 32 GB would be better - incidently I have 64 GB and I find that helps with some programs such as SAI/Painter/Harmony - Chris is right to point out the power increase though; it is crucial that your power supply can cope with all of your upgraded components with a margin to spare, and like any kit with fans make sure you buy quiet ones
graphic card-nvidia geforce gt 720
this is definately weak, some rendering programs will crawl - mine is GTX 1070, and you will need that sort of GPU power at least, sooner or later for any serious rendering; some of the higher spec graphics cards are also big beasts and will not fit in small cases
monitor-hp w/2207 native resolution 1680 x 1050 at 60 hz. Probably 5 years+ old
Personal preference indeed - you can go into respose times and colour gamut, and the display technology, but at the end of the day, if you do not need to fit into a team, your eyes and your viewing are your guide

monitors are not part of the computer, so you can deal with them seperately, which brings me to what paul said, and i would agree that you should not spend money unless you have to

if your computer is getting unacceptably slow then you can then think about upgrading, meanwhile the best move you might make could be a second monitor, once you see the benefit of having your drawing area on one screen and the dialog and option boxes on the second you may never wish to go back - but note, can become a pain if the two monitors are not the same native resolution