By the way, you have until April 15th of next year to complete your Roth Contributions for 2012.
Also, what is interesting is you have zero exposure to international securities and bonds.
Bonds are tricky right now. But, they do help to reduce your volatility.
Also, you should have some international exposure. Here is piece done by Vanguard which explains diversification benefits of international securities. https://personal.vanguard.com/pdf/icriecr.pdf
Most important, at least to me, is figure 8 which shows the differential between US stocks and non-US stocks. As you can see, there are times when US stocks dominate and times when non-US stocks dominate. The point is, when you "buy and hold," it is always implicitly behind the scenes that you also rebalance. So, with figure 8 starting to head towards US stocks outperforming the rest of the world, guess what I am selling and guess what I am buying? This illustrates the point of how diversification, even bonds, can help to reduce the volatility of a portfolio and give similar, if not better, returns IF YOU REBALANCE.
As far as how much you should hold in int'l securities, the Vanguard piece answers that in the conclusion,
In light of quantitative analysis and qualitative considerations, we have demonstrated that domestic investors should consider allocating part of their portfolios to international securities, and that a 20% allocation may be a reasonable starting point. Although finance theory dictates that an upper asset allocation limit should be based on the global market capitalization for international equities (currently approximately 58%), we have demonstrated that international allocations exceeding 40% have not historically added significant additional diversification benefits, particularly accounting for costs.
I would also suggest the Boglehead's investing book (at your local library).
You have a good start. The final thing I would add, have some fun as well.