Your budget rules out the 135/2L.
The 50/1.4 would be an improvement over your 50/1.8... but not for the slightly larger aperture you mention. It's got faster and more accurate USM focusing (tho it's actually a hybrid form of USM), is more resistant to flare and thus gives nicer contrast and better color saturation, plus has nicer bokeh thanks to more aperture blades (8 vs 5 in the f1.8 lens). Both the 50/1.4 and 50/1.8 are better stopped down slightly, a little soft wide open... so the 2/3 stop larger possible aperture really isn't all that big a gain. I tend to use my 50/1.4 at f2.2 or a little smaller, when sharpness is important... of course, it's nice to have f1.4 when I really need it and can tolerate a little softness. The Sigma 50/1.4 is sharper wide open, but a little more expensive and a whole lot bigger and heavier than the Canon.
Your choices for somewhat longer primes really come down to the 85/1.8 and 100/2. Pick the one you like best, both are within your budget and are good lenses. They are both USM lenses, fast and accurately focusing, certainly faster and more accurate than your f1.8 and more comparable to the 50/1.4.
Don't know about the third party 70-200/2.8 zooms... but at best they are f2.8. The Canon 70-200/2.8 IS II might be the ultimate, but costs about four times your budget... and is still "only" f2.8 (though it is usable wide open). You might find a used, original 70-200/2.8 IS for less, but it still will likely be more than double your budget unless you find a screamingly good deal on one. There are the non-IS 70-200s to consider, too... They might work, but IS is sure nice to have (even with sports) when you don't have f2 or f1.8 or f1.4 in a lens.
