I have a G1 right now and for starters, it's still on Adroid 1.6. The phone itself does not have a lot of memory and it lags most of the times. If you have too many Apps open at once, the phone will be very slow and force close on some apps and freeze others.
Therefore, you have to download an App such as Taskiller or Advanced Task Killer so that you can kill the applications you don't want open and that will help you phone run faster. Throughout the day tho, I kill all Apps about 5-6 times to keep my g1 running at it's fastest.