Well, here's my investing advice... take it for what it's worth.
I started investing when I was 21 (just shy of 24 now), shortly after I had gotten a full-time job with a 403b while I was still a student. (I work at a university.) I did a ton of research and reading before I started (along with the other books already mentioned, check out the classic
A Random Walk Down Wall Street), which helped me develop my index-only investment plan.
One of the most important things in investing is to not let your emotions become involved. Of course, this is also one of the most difficult things about it. Everybody hurts when they see their investments dropping in value. That's why developing a solid investment plan is so important. You have to understand your tolerance for risk, and then invest accordingly. This won't make the pain of seeing a declining asset base any less, but at least it won't give you a heart attack because you knew this was possible. Having a clearly dilineated plan promotes discipline -- that's why setting up auto-investing is so useful. It's a lot harder to write that check to buy more shares when you see your folio dropping every month.
I'm not a fan of any funds that have loads, so I don't like AGTHX. That said, I wouldn't pull out just because a fund has been declining. That's not a good reason to jump ship, tempting as it probably is. I think you should really pause and develop a clear investment plan with goals and risk tolerance before going any further. Doing so will probably help you answer your own question.
As for diversification: It's always good to be as diversified as possible, but given your limited funds, you can probably wait until you're working full-time before adding more diversification.
Have you ever looked into index investing btw?
As has already been mentioned by a few, checking out
morningstar.com for a ton of good, helpful advice. It was an invaluable resource for me when I was first starting out.