Sorry I'm late getting in on this one to, but to me it already sounds like you have what you need to get started. The two reoccuring themes in lens choice for wildlife and animals is longer and faster. My little Sony H1 has something like a 432 mm equivilant lens and even with that, I often find myself pushing on the digital zoom occasionally. 500-600mm would be ideal for me...I shoot mainly at zoo's and nature centers. In good bright light, what you have should be ideal (like a nice sunny day out at the zoo for example). If you're looking to shoot in less than ideal lighting situations you may find in time that you're going to need a faster lens as well. Let's face it...most critters tend to move around a bit. Even if you're using the most solid tripod on the planet, it's no guarentee that the critter's gonna stand still long enough for a crisp shot. Even in bright daylight, because of the long lens chances are you're loosing a couple of stops of light with those long focal lengths as well...low light equals longer shutter speeds and when you combine that with a subject that won't stop moving, together that equals bad/blurry pics. I'm guess that Sigma's only like an f/4.5 at best...you're going to have to make up for that loss of light with slower shutter speeds...it's something to keep in mind. The problem is that really long, really fast lens are also really
expensive (I've priced a few in the $8000 range!).
The actual number of lenses that end up in a person's arsenal is really subjective based on the individual. There are people out there who prefer one good all-purpose zoom lens while there are other's that carry around an infinite number of prime lenses so they have "the right lens" at all times. Using myself as an example, I shoot mostly critters, but do the occasional landscape or macro shots (mainly flowers and a few bugs). When I go into DSLR, I -know- I'm going to need 1 really long lens (as fast as my checking account with allow), one medium all purpose lens and one good macro/wide angle lens. Being the artistically driven person that I am, I also may eventually invest in a specialty lens or two...something like a "Lens Baby" or something similar. Other then that though, since my main interest is critters/wildlife, the only thing I see myself wanting is just longer and faster...if I can shoot a pic of a cheetah at a full run from half a mile down the road around sunset/dusk and still have the pic come out bright -and- tack sharp....then
I'll know I'm on to something! LOL!!!
The last thing I would consider is simply how
you intend to shoot. I was watching a documentary on National Geographic Photographers a while back and the one thing that has really stuck in my mind was the guy taking a picture of a female African elephant...the heard Matron. In a nut shell, this thing was charging the guy and he said in his own words that the only thing he was thinking about while taking this shot was "when to run". After he clicked the shutter, he and his assistant took off like a bat outta hell as this elephant charged after them. He said while he was running he was wondering if the picture was in focus, was it exposed right, etc.. This is where it pays to have something like that $8000 Canon lens...you have
to get the shot (lets face it, we're talkin National Geographic here!) and you don't have a lot of time to worry about focus, f/stops and shutter speeds. Now in my case, again I shoot mostly at zoo's and nature centers...if I don't get that great shot of say the lion's for example, I can always try again my next trip. No pressure, no big deal...and no real reason to blow $8000 on a camera lens either. See how it all works?
I would say for now, learn to work with what you have...and you have a good start there. Work on things like composition and understanding how what you have actually works (if you learn to work effectively with the cheap stuff, you'll apreciate the good stuff that much more when you get there!). Worry about getting more lenses when you've got more experience under your belt. Time and practice will eventually dictate what you'll actually need