I use NT's performance monitor from time to time for just that purpose. Yes, it does have an effect on performance, just like any application would. How big of a performance hit will depend on what you're monitoring and how often you're monitoring it. I haven't used System Monitor.
At any rate, to get an ideo of what's going on with your system, it's typically ok to sacrifice a minor bit of performance to the application doing the monitoring. You'll still be able to clearly observe the various statistics *changing*, which is really what you're after. Plus, a monitor is probably only going to cost a few CPU percentage points and a few kilobytes of memory....which of course can add up to quite a bit when you're running critical realtime applications (e.g. recording).
In other words, try it.
Slackmaster 2000