will this be your first amp/rig??
are you in a band, or playing at home alone??
how much money can you spend or how quickly to you expect to upgrade to some thing later??
if you're just starting out and learning at home and don't have alot to spend. I'd probably recommend a combo. easy to hook up and use. easy to move if its low power (0-60w).
if you want to play with friends, but don't have alot ($1000+) I'd still recommend a combo, but suggest at least 100w 112.
if you definitely need serious power and will be with a band separates is probably the way to go, if you have the money. I'd recommend a head (pre+amp in one case) at least 400w into 4ohms. at least one 112 or 210 cab 8 ohms, another cab if you can afford it.
if you can't spend alot, but still need decent quality volume for a band situation, you might consider 200w+ combo with 112 or 210 with the option to add another cab to the speaker output. or seriously check used equipment and pawn shops, but try the equipment out to make sure it works good.
brands I'd recommend checking out...yorkville/traynor, avatar cabs, eden/nemesis heads (blue),
and most of all SEARCH talkbass about brands you are considering and then ask any left over questions, also check the TB for sale stuff....