Bass amps require a higher wattage than guitar amps to achieve a similar volume/sound in general.

if so why and to what degree?
Yes, they do. It's because of the way the lower frequencies travel or something of the sort.

To what degree? I've heard there's a rough rule before, like the bassist would want around 2x what the guitarist has...can't quite remember.
Thanks alot, I'll bear that in mind when i buy an amp.

EDIT: If anyone does know the aforementioned rule, please post.
It all has to with speakers, almost all speakers are not very efficient at Freqences below 100-80hz, since bass gutars primaly play between 40-100hz, you see how this can be a problem. Subwoofers, and loudspeakers used in bass cabs and amp are typically 90-95 db 1watt@1Metter, 40-500hz and typically guitar speakers are 97-102 dbw/1watt@1 meter, 70-5000hz so you have a big volume difference right at the start. But honestly if your wanting a decent bass amp look for somthing with 150-300+ watts if your going to be giging, 300-Plus without PA subwoofers, with soild state amplification. With a bass tube amp I would recomend 100-200 watts, that should really suit you well in most situations.