Do I need to upgrade to Windows 10 and is it worth it?
In my opinion; no, it's not critical. You can do several things to keep your older PC running and safer online without throwing hundreds of dollars at an old PC. First, make sure you run a web browser that is still receiving updates (i.e. - Mozilla Firefox or Google Chrome). Second, install a third party antivirus (AVG, Avast, Bitdefender, Symantec/Norton, etc.) and the free version is usually fine for personal use. And finally, be aware of where you go on the internet, that's the most critical.
Be conscious of what you click on.
It sounds silly, but it's that simple. When you look at most websites today you will notice a small "ad" logo tagging certain articles or images. These ads redirect you away from the original website (MSN, Fox, Yahoo, etc) to a third party site.
Also, when you install "free" software please read the user agreement and the "optional software" that may want to be installed. Most of those utilities may not be outright spyware but may be downloaded from an infected source.
For many people the answer, unfortunately is, no it is not.
With the pace of technology it's quite possible to replace your computer for a few hundred dollars. And for most people that simply surf the web this is the most practical way to upgrade.
If your needs are more robust, say intensive gaming or CAD/CAM work then by all means look at the options to improve your existing system. To get the most out of your particular application may require a simple hard drive swap or video card upgrade. This would be far more cost effective than replacing your entire high-end system.