Here's one of our two new kitties!!!
Took this with my 40D w/50mm f1.4 @ f1.4 ISO400. Notice the small strip of focus area through the shot from the couch, across the carpet, and to the kitty.
IMHO, unless you are going to get the 70-300 with IS, the good one, don't bother. The low-end 70-300 is really really cheap feeling and not that great glass. I would go for the 70-200 f4.0L @ around $500. This is similar in cost to the descent 70-300, but WAY above it in glass quality. The 70-200 f4.0 series is said to be one of the sharpest long zooms available.
You should absolutely love that 50mm f1.8. I almost got that, but had the extra jack and went for the next up. The only difference, besides the .4 fstop, is the bokeh. With the f1.8 your bright DOF spots (like the one just to the right of the kitty's head) will show kind of like octagons because there are less shutter blades. It really makes little difference and the f1.8, from my readings, is outstanding at under $100.
BTW, what body are you shooting?