A few things I wanna add:
If you go 6505 get a used one. Peavey recently started manufacturing them in China so I would keep an eye open for a used head and make sure the guy selling it sends you a picture with the Made in USA tag. It's not like the Chinese stuff has to be unreliable, but why risk it?

Anyway, pretty much everything from the 5150/6505 range will give you more gain you'll ever need, keep the preamp gain low and add a Tubescreamer to tighten things up. This will BURY your fellow guitar players Powerball in a mix

Other heads I can recommend and have first hand experience with:
As already mentioned here take a look at the Jet City range. Dirt cheap, amazing sound and great bang for the buck. Will need a Tubescreamer for ubertight riffing though.
If you're not going for the ultra modern downtuned Djent thing a used JCM2000 DSL 50 or 100 Watt head will yield great results as well. Very nice midrange bite and a pissed off rock n roll attitude but can get very heavy, again with a Tubescreamer. Just got myself recently one of those puppies and I'm more than happy. Great clean sound as well

An EVH 5150 III 50 Watter can be had used as well for the price range you're considering. Amazing sound. Might have some reliability issues, so let a tech retube, bias and check it.
Honorable lunchbox mentions:
Laney IRT Studio, Orange Tiny/Dark/Dual Terror, EVH 5150 III LBX, Peavey 6505 MH.
Hope that helps, if you have more questions, fire away!
Cheers