it's a good idea, but it depends on what you do with your computer. if you.. 1) play games, 2) have lots of programs open, or 3) use single, large programs (photoshop, etc).. then i would say it's a good idea. but if you just mostly surf the net or whatever, i wouldn't worry about it.
also, if you have things like anti-virus running in the background. those eat up a good amount of memory and you will probly run short with 192mb.
you can also open task manager and, under the performance tab, you can see how much "physical memory" you have available at any given time. if it ever goes below, say, 30mb then i'd upgrade. i guarantee that if you do any of those things i mentioned before, that 192mb won't be enough for smooth performance.
make sure you have a free memory slot, though. or else you might be replacing the 64mb stick instead of just adding.