You need 300+ watts to be playing seriously. A good rule of thumb is that a bass amp should be 10x the wattage of the guitar amps you're playing next to, since it requires about 10x the wattage to move more air. Don't take this to mean that you need 1000 watts of bass amplification because everyone buys 100w Marshalls that never get played to their potential because they're too effing loud. I have played thousands of shows in my life as a bassist, and my 350w SWR has been with my on the majority of those with no complaints. Just do yourself a favor and get something well made and reasonably powered. Don't buy cheap Peaveys or Fenders. Get a decent bass amp. I encourage you to get a decent head and a 4x10 cab to start and add a quality 15 or 18 if needed.
Modular is better than combo, and quality is always better than cheaper. You'll hate (and destroy) a low end bass amp very quickly, whereas cheap guitar amps can sometimes sound "cool" because of the tonal range of the guitar. I can't stress enough how important a quality amp is to your bass sound.
-Mark