Don't even think about trying to shoot volleyball without an SLR. Early on I did (I had a Nikon 8800). It just doesn't work. Your two biggest problems are going to be slow focus speed, and too much depth of field. So, forgot about your cell phone camera, and forget about your pocketable P&S (note that at the time I write this - Feb 2010 - there are new cameras coming out that are mirrorless but with interchangeable lenses. I don't know if these will be suitable for sports photography or not)
Don't think you need to go out and drop $8000 on a body and lens to get decent results though. Quite the opposite, I think you can get top notch results with kit that retails for under $1000. You'll have to make some trade-offs of course, but it is doable.
Consider, virtually all of the shots I took from 2006 to 2009 were made in single-shot mode. My camera was capable of 5FPS, but while I experimented with machine-gunning the shots I found I had more consistent results if I relied on my own timing. Eventually I imagine we will all be sporting 1000FPS cameras, and when that happens it will be a sad day for photography. For now though, if your camera is <7FPS learn how to time your shots!
And don't worry about needing some mega-boss number of autofocus points. Without a doubt more helps, but you can certainly get by with just a single cross-type center autofocus point. For many types of shots you don't need to rely on autofocus at all. Tailor the types of shots you attempt to the auto-focus capabilties of your camera.
Megapixels will determine your maximum print size. If your target output is the web, then pretty much anything will work. 6Megapixels? No problem. You can crop half the image away and still have a perfectly good websized image. If you want to print then look at 10MP or greater.
So for a body get whatever you can comfortably afford. I shot briefly with a Nikon D70, then a D200 for four years, and as I write this I am waiting for the 2010 volleyball season to start to try out a D300s. Personally, unless you are a professional or independently wealthy I see little reason to go beyond a D300s or a 7D.Lenses are a different story. Unless you want to spend hours in photoshop creating masks and applying gaussian blurs you are going to need a relatively fast lens. Fast lenses are, generally speaking, not inexpensive. Fast lenses that zoom can be downright pricey. This is where the tough decisions come financially. When push comes to shove, sacrifice the zoom before you sacrifice the speed. A fixed-focal length wide aperture lens can give you great photos in certain situations. A slow zoom lens will never give you better then good results. Depending on your budget consider one of the 50 1.8, 50 1.4, or 85 1.8 (a used 50/1.8 can often be found for less than $100, so if you bought a lower-end body used as well you could come in under $500 total). GOing up from there you could look at 70-200/f4, or older versions for 80-200/2.8. If money isn't an issue then just jump straight to a 70-200 f2.8.
Other than a body and a lens I don't think you need anything else. Certainly not a tripod or monopod. I've experimented on and off with polarizers and ND filters to allow me to shoot at near wide open while still keeping the shutter speed at ~1/500th, but we will address that later.
Let's do some rough calculations here. Let's presume that Sean Rosenthal is on offense and is going to bring the heater. And just for giggles maybe he decides to hit with his off hand. So the ball will only be travelling about 60MPH. 60 miles per hour * 5280 feet/mile * 1 hour/60 minutes * 1 minute/60 seconds = 88 feet per second. Now, if you have a camera that shoots at 5FPS how far will the ball travel between shots? 5FPS means a shot ever 0.2 seconds, so 88fps*0.2seconds = 17.6 feet. ~18 feet is a lot. If you have framed your image so that a 6'2" player just about fills the frame vertically then you have a horizontal field of <5 feet. I'm not gooing to make any time estimates for how long it takes for the ball to ricochet off the defender, but let's presume it is 'quick'. This means that if the ball is just outside of the frame when your first picture is taken that the ball can travel into the frame, off the defender, and all the way back out again before the second image fires. If Sean hits with his right hand, or if your camera is slower than 5FPS then the situation is even worse. 7fps = ~.14 seconds between exposure = ~12.7 feet per exposure 8 fps = 11 feet. 10 fps = 8.8 feet Now, you can certainly get good shots where the ball is not in contact with the player. And if that is what you want, then go ahead and shoot at your fastest frame rate and sort through the results and see what you get. But if you wnat to get the ball smushed over the defenders arm, you are going to have to rely on technique, or wait until cameras capture at 24FPS. Now there is a situation where the extra FPS helps. And that's the diving defender. Attempting to time those shots is next to impossible.