amps are really costly, so practically you're not going to get a very loud one for $400. I know Vox has a pretty good 23 Watt amp that you'll get for under $400, and the effects are pretty nice. obviously it's not too loud, but depending on the type of music you plan to play it could be fine.
http://www.guitarcenter.com/Peavey-V...93-i1413609.gc
good amp, good deal, and that one's definitely loud enough for ya. I know peavey has some other good tube amps that you'll get for a good price. Plus Sonic Youth use peaveys.
if you can find a good deal on one that's in your price range, a Fender Blues Jr. would also be a logical choice