Found this app made by DinarSoft©, called HandySwitcher® that do exactly what I've been looking for. Unlike others same apps, HandySwitcher v3.0 is very light and run simultenously in WinMob system, it doesn't take up too many of Treo's resources. You also can put HandySwitcher on the Today screen, this really shorten the time you need to close any apps in order to release memory that's been taken up. Another feature I like is that it can force to close all the apps running on the background, and if you want to you could keep the one that currently in use/active. There are others small features such as informations on CPU & memory performance available in your Treo, read on their website here to find out more about this application.
I recommend HandySwitcher for those who use their Treo 750 for heavy multi-tasking, this is how a Task Manager app should be made.
(blogged with my Treo 750v)