1) Why is it important to have a good router?
I wouldn't say it's important, but it's very nice to have. A good router will reduce the latency of connections due to better network interfaces and faster processing, meaning that web pages will open more quickly and give you slightly more of edge in online gaming. They can also include under-the-hood features and more powerful signaling which can make your connection faster if you're on ADSL (not so much for fibre).
Case in point - kmanmx got a new router a couple years ago for one of his parent's home connection, in order to improve the throughput. While it did nudge it up slightly, what actually ended up being the big improvement is how much quicker and nicer web browsing was due to the pages loading more quickly.
2) What should I look for when buying a new router?
I'm not really sure as I haven't done the research. I'd talk to kman, I think he knows at least a bit more than I do. I do know that it's not worth buying the £100+ routers you see, as at that point you're just paying for software/convenience features which aren't much use if you're not a basic user.
3) Why isn't my ISP Router good enuf
Because they try to make them as cheap as possible will still being functional. This means that they die more easily and more quickly, they aren't as fast as they could be, and they are MUCH less stable on average, meaning you have to reboot them quite often. They also run hotter due to cheaper design. A decent router should need rebooting maybe a couple of times a year.
4) should I upgrade from the standard Router I have to something like CISCO.
No. Just get a good consumer router. Cisco brand is for large organizations and you will paying for up time, quality, and features that you could never make use of, so it is a waste of money.