a few DSLR basic questions...

So i've been eye'ing the 40D for awhile now. Right now im using a Canon S3IS

I have been doing alot of reading and figured out quite a bit, but i have a few questions i need some help on, btw, budget is tight at this point!

1. I LOVE the 12x optical zoom on the s3, i think thats 432mm according to DPReview, so to get an equivelent lens, would a 200~300 lens be less than that?

2. for low light, weddings, concerts etc, i really want a lens with a fast aperture. How fast do i really need it? is f4 too small or do i need to go bigger to f2.8 or more? the low noise at high ISO on the 40D should help get me a faster shutter speed right?

3. i see some lenses like 50mm - f2.8... is there something special to these? why would you want a lens with no way to change the zoom?

i'm sure there are more but i will get to them later, any advice is welcome, and thanks in advance

Welcome to the potential world of DSLRs... Like you, I "graduated" to a DSLR from a Powershot S3IS. If you're anything like me, once you get the DSLR, your S3IS will collect dust!!

ANyways, to answer your questions:

1.) The 40D has a 1.6 crop factor with it, so to reach the 12x optical zom, or 432mm focal length, you'll need a lens of 270mm or greater... If you can get a zoom lens like the 70-300 you'll actually "see" more zoom than what you did with your S3.

2.) Some folks manage to get by with an f/4 for weddings, but I'd say the majority of folks use/require at a MINIMUM f/2.8. Fast primes in the 1.2, 1.4, 1.8 range are much more suited as you can shoot with an even lower ISO.

3.) Fixed lenses, aka prime lenses, offer several advantages to zoom lenses. First, the glass is tailored to a specific focal length, so you'll see generally sharper, higher resolution images versus a comparable focal length on a zoom. Prime lenses also offer much larger apertures- ideal for weddings... I think the fastest a zoom lens will go is f/2.8 while the fastese production Canon prime lens is 1.2... thats a HUGE difference in required shutter speed...

I hope this helps!