Does signal strength affect download/upload speeds?

"Yes and no... (depending on both stick and service)

If you are talking about the difference between five and four bars, you are not very likely to see a difference, however if talking about one and two, it can make a big difference.

The only way to really tell is to use a speed checker or the utility that comes with the 3G card and tell.

Typically on the newer faster sticks (7.2MB) quality can make a big difference. On some of the slower ones (~2MB) I do not see much of a difference from 2-5 bars. Again, it depends on many other factors."