Good guitar for under $500...not going to happen. Save money until you can afford a good guitar as a quality Guitar is an investment. If you like the SG. Save until you can get one.
I'm not saying you have to spend PRS money...but you should be able to save enough for a $500-$800 ESP Viper (their SG type) or ESP LTD EC-1000 (Their Les Paul styled axe).
For metal, I always recommend ESP/LTD, Ibanez, Jackson, or PRS (the SE models). They're great guitars, and you should be able to find a good one if you take the time and give yourself a decent budget. And the plus side is they're about $400-$500 cheaper than a Gibson.
I use Ibanez and ESP on stage and in studio and they're real workhorses. Great guitars for decent prices.
Some people would even recommend Schecter, but I don't really like the necks they put on their guitars (I owned a C-1 that I traded for my Ibanez S770.) I thought that the Schecter neck was like a 2x4 with strings on it (Others say it's like a Baseball Bat).
Just go to guitar Center or Sam Ash and play until you find what you think feels good in your hands and sounds good to your ears. We can throw around models and brands all day, but it really boils down to personal preference.