How "Loud" does a Guitar Amp need to be?

Just Read On. I been playing acoustic guitar for about 2 years now. I now want a electric. I have been borrowing my friends 20 watt amp. I know the wattage has a direct relation to the loudness. i need an amp that will be loud enough to jam with my friends. I have full band (drummer, singer, bass and guitar which is me) and i play in a basement. I want to get a vox Valvetronix+ amp because of their Incredible amount of Effects. These amps come in several option. Vt20,Vt40,Vt80,Vt120. The vt20 is already out of conversion , i dont want this. But the other ones i do. I need to know what Wattage is righting for playing so i can decide on which amp i need. THANKSSSS