#1
I've been playing for around 4 1/2 years now and I have a decent guitar but my amp is only a Fender Frontman 15G. I decided it's definitely time for an upgrade. But I don't really know what to get. Should I get a cheap half-stack or a combo amp? Is there really going to be a huge difference between solid state and tube amps? I need an amp I can use for small gigs like parties and small venues/theaters and my budget is around $500, though I can go a little higher if needed. I'm looking for something good for Progressive and Metal mostly.

Bands like:
Protest the Hero
Mastodon
Coheed and Cambria
Iron Maiden
Rush
Megadeth
Metallica
Black Sabbath

Any help and/or suggestions would be greatly appreciated.
#3
All depends on how much you're concerned with the clean channel, but a 5150/6505 will definitely get the metal sound down good.
Also, JCM's are pretty good too, check'em out.
Guitars
Ibanez Prestige RG1570
Schecter Omen 6
ESP LTD Viper 400
Dean Dime From Hell

Amp
Peavey 5150