CBEffects is quite fast and non-memory-intensive, as particle systems go. In fact, recently, as I’ve played around with making a new particle system just for fun, I’ve found it difficult to reach CBEffects’ performance. The number one thing to remember is… Don’t try to optimize things unless they need it. Put another way, Premature optimization is the root of all evil. Unless your game is running slowly and lagging, I wouldn’t worry about it.
And 2D games’ memory levels are usually far lower than 3D - even with a lot of animations and everything running, it’s difficult to get too much memory. I can’t give an “average” or “normal” memory usage, because it varies drastically between applications. As long as your app doesn’t lag or crash because of memory usage, you should be fine.