Don't put a task killer on the phone (at least not yet and especially not an automatic one), you don't really need it unless an app gets out of control and you can kill those from the settings menu.
The power saving widget is good, but in general don't worry about turning GPS or wifi on and off all the time to save power. The GPS only comes on when an app needs it, so it's not a huge power drain unless your running something like maps in foreground. If you're wandering around and don't have a fixed wifi connection, leave it turned off, but at home it will actually save battery to use wifi instead of 3g (or at least be a wash).
Get the BatteryLeft widget and the xScope browser. BatteryLeft will tell you, after it has learned your usage patterns, when your battery is going to say goodbye (of course, Murphy can tell you that too... the worst possible time

). xScope is a fast web browser with multitouch and a built in file and task manager.
If your email comes from somewhere other than gmail, I recommend two options 1) Have gmail pick up the mail from your other accounts (this is what I do) and present it in your gmail account or 2) The K-9 mail client is highly recommended over the stock email client.
Finally, explore the market, there's a lot of good stuff out there. Fun stuff, like the tricorder app and a bajillion games, Wow! stuff like like google earth and google sky map and just practical stuff like Evernote.
Oh, and don't worry about rooting the phone or overclocking or any of that stuff just yet. Get comfortable with the phone, learn what it can do (play with it!) and most importantly have fun!