I got this all the time when my Charge was new. It's been a while, but I believe it was specific to background task memory. I tried the clear memory button, which only helped temporarily because all the background services would eventually restart. So when given the option to not get this notification any more, I said "yes". Don't know if it caused any side effects by ignoring that, but I figured I didn't really have any other solution.
There has been a system update since then, and I've noticed that the amount of background services memory available is much larger. (Home - Settings - Applications - Running Services) Don't know what changed, but I'm guessing that the update increase the allocation for background services.
I didn't record the previous usage, but right now for example, I show... "Available:41MB+83MB in 17 services."
The only time I really notice a lag (other than expected moments like when an app is installing, or some other heavy processing is occuring) is when I am on the built in launcher. I plan to try out Launcher Pro or Go Launcher EX, but never seem to get around to it.