Another good piece of kit, apart from an OIII or an UHC filter is to forgo looking through your eyepiece and purchase a GStar camera. They're a very sensitive, low light video camera designed for astronomy that allows you to see things directly that you normally wouldn't see through an eyepiece. You can also use them to take piccies as well
If you're interested, goto here...
http://www.myastroshop.com.au/products/gstarcam.asp. Steve Massey, the guy that owns and runs MyAstroshop, will help you a lot with anything to do with the camera (anything to do with anything, really) and he's a great guy too
. With the GStar, objects like the Crab, which are buggers to find at the best of times, will be easy targets...the Crab shows up very clearly onscreen and you can even see the pulsar powering the neb' at its centre, as well. Your 6" will have no trouble doing this with the camera attached to it. Actually, the camera will allow you to see objects that you normally wouldn't see in your scope as it will virtually double the aperture of your scope....you'll routinely see stars down to 15-16 mag through your scope with the camera, even in moderately light polluted areas (your scope should be able to pick stars down to about 12.5-13 mag without assistance at a reasonably dark site). They come in two flavours, mono or colour and for their price, I think they're great value. I own one myself, JJJ has one and there's a few more of us that also own a GStar. There's also another camera called a Mallincam that is of the same type as a GStar...can't remember who owns one here, but I know one of the guys here at IIS has one...but it's more expensive than the GStar and they're Canadian. Another thing they're great for is when you have family and friends over. You can connect a GStar upto a TV/Monitor and have a little "show and tell" session in your backyard!!!
Great for school outings and star parties too
. So, think about it, have a talk with Steve and the others here that own one. See if it fits the bill for you and go from there