The D90 takes the EN-EL3e battery, same as the D200, D80, D300, D300s and D700.

The D90 takes the same battery grip as the D80 (MB-D80).

Most of the time I don't use a battery grip with my D300. It makes the camera too big to carry in my favourite bag plus I've never managed to get beyond 50% discharge in a full day's shooting. I use the battery grip only when I'm using big lenses and I need to hold the camera vertically.

The only filter you need is a transparent one to protect the front of the lens (optional). The arty effects stuff you can do in post-processing (you're going to need processing software). You do need lens hoods but these may be included with the lenses.

The major accessory that you need and it's an important one is a cobra flash like the SB600. The built-in flash on the camera is useful for emergencies but it doesn't really go out beyond 10 feet. Now you've got a nice camera people will expect you to take the photos at family gatherings and weddings and stuff. The Nikon CLS flash system is excellent and it makes things much easier.