To #1. By slower do you mean 3 seconds slower or .03 seconds slower (numbers as examples)?
By gearing, what you are doing is changing the torque/rpm ratio. For example, if you have a gearing ratio of 300:1, then your ratio changes to:
(torque*300) / (rpm / 300)
So in effect, your torque multiplies by the ratio, and your speed divides by it.
Adding gearing also reduces efficiency. For example, if you used a worm gear, multiply your output torque and speed by .8. This is from internal losses such as friction and force angles.
I actually did a tutorial on robot gearing awhile back, if your interested:http://www.societyofrobots.com/mechanics_gears.shtml
Hobby servos probably wont work for you at that weight. You will need to add some type of feedback control to each motor. Perhaps use rotary potentiometers at the joints?
I just did some rough math, sounds like your gonna spend $1.5k+, right?
I have plans for a biped in the works too, but that pricetag isnt far from the one I calculated for mine . . . bipeds arent just inherently hard, but inherently expensive too . . .