Lucky8 said:
50mm f/1.4 built quality is a lot better than 50 f/1.8 plus USM
it's about 4X more expensive.
the only relly bad thing about the 1.8 is its focusing motor. It's fantastically sharp and fast (1.8!)
the sigma 18-125 seems like an excellent value at $250-300
the sigma 18-50 is a better, but more expensive lens than the canon 18-55
the 50mm 1.8 is great, but at 80mm equiv, is pretty long for a lot of things
i love mine, and the 1.8 has saved me several times. The spped, sharpness, etc is a fantastic bargain at $50-70, and shooting with a prime like this one has two effects:
1)improving your photography, because you think more about where you are relative to your subject, and the focus, where ppl with an enormous zoom are prone to staying in the same place, playing endlessly with the zoom, ignoring other aspects
2)making you jealous of all those pretty L lenses with their aperatures, and the solid build.
-worth the risks, if you ask me... I reccomend the 50 1.8
I got a 55-200mm for $100 new on ebay when they came out. It's not great, but more than I was expecting for $100
try to get a feel for the kind of pics you take... a zoom is great, but a prime is small, light, and fast. telephoto is a whole diff realm that is as limiting as it is enabling
then, get some combo of:
18-50 or 18-125 sigma
50mm 1.8 canon
possibly a cheap tele-zoom like the 55-200 canon
p.s. i'd buy the lenses before the filters. a UV protecting filter is good as far as dust, scratches, etc are concerned, and is much cheaper than a polarizer
Edit: it may be too late, but i feel that you may be limiting yourself severely by getting only the 50mm!