Mike,
Maybe the best way for me to offer my opinion is by starting with what
I use day-to-day. I always have two Canon 40d's ready to go in my
truck. One has either a 16-35mm f2.8 or a 24-70mm f2.8. The other
has a 70-200mm f2.8. I pretty much only use "L" Series Canon lenses
these days. The only thing I find annoying about "L" Series lenses is
that there can be gaps in available focal lengths from lens to lens,
requiring you to have different lenses on hand like the ones I've
mentioned. The "L" Series lenses are great lenses and can be very
pricey so if you're just looking for a decent all-around lens to have
fun with you may consider Tamron, Sigma, etc. if you're on a budget.
These days, we're pretty much ALL on a budget.
You mentioned you wanted speed (aka f2.8). You'll be hard-pressed to
find a sub-$800 lens that will get you that speed. Personally, I'm
considering the Canon 24-105mm IS lens as an Xmas present to myself.
It's relatively fast and covers a great range in the way of focal
length. Much better for day-to-day shooting in my opinion but I
haven't really had the chance to play with one yet. The painful part
is that they're running about $900 new.
I always advise that while you can go cheap on the camera body you
should never go cheap on the glass. Pretty much any Canon DSLR will
make a great image if you let it. If you have a cheap piece of glass
on the front of it, that alone could ruin what would otherwise be a
memorable shot. How many photographers upgrade camera bodies on a
regular basis? Plenty. Why do they tend to stay with one
manufacturer? A big reason, at least for me, is because I (they) have
invested too much darn money in good glass to have to go buy all new
stuff!
One more thing, when looking at consumer and pro-sumer Canon lenses,
if you see the option to get a USM lens vs. a non-USM lens - spend the
extra money for the USM lens. They'll look exactly the same sitting
in front of you on the counter but the USM lenses have much better
components, especially the focusing motor. They're much quieter and
faster to lock focus. You'll thank me later.
I've had great luck on Ebay and of course at Samy's (http://
www.samys.com) and B&H (
http://www.bhphotovideo.com). You'll normally
find better pricing than your neighborhood photo shop unless, of
course, you want one-on-one sales service. Also look for rebates.
Final note - You're 18-55mm should work well for you in general, but
with the smaller sensor of the XTi (as with many of the current Canon
DSLR's) the 18mm becomes the equivalent of a 28mm so it may limit your
wider shots. That same magnification helps you on the other end
though. The 55mm becomes an 88mm and you don't lose any light in the
process. Long story short, what you really have is a 28-88mm lens.
Don't go spending all kinds of money on glass unless you know for sure
what you already have isn't satisfying your needs. Varying your
technique can also help you get more out of a lens that may otherwise
cramp your style.
Hope this helps! Good luck!
Craig D.