My 3GS starting freezing/acting up a few days ago. Having redone all my folders a few days prior to that, I had the bottom tray full.
I took every single one of them out of the tray, and the phone was fine again.
What struck me as strange about this, is that very few of my apps have any b/g multitasking...they were just sitting in the 'recent' tray doing nothing.
They shouldn't have been an issue.
Are all the apps you have down in the task bar actually iOS4 tested and compatible. What I found was when I had a few of those running that were not compatible it not only took my battery life down, but used up system resources. However, when I have apps that are iOS4 compatible I don't find it necessary to shut them down.
For myself as a general rule. I have this habit and it is totally unnecessary but I do it because that is just me. When I charge my phone at night, I close out all the apps and restart my phone. This way I know when I hit the ground running the next morning my phone is good to go with all its resources for the day. Again not really necessary, but it just something I do. I have extremely great battery life and my phone runs perfectly all throughout the day. In fact where as my 3GS use to be at around 30% battery left at the end of the day. I am finding my iPhone 4 is at around 50% or more. So, I am loving that.
My son has my 3GS and he does what I do each night just cause I told him I thought it was a good idea. He is very pleased with his battery life and performance too.