Can you trade comodities such as Gold, Silver, Oil, etc using an online brokerage and a Roth IRA?

My online brokerage is TradeKing. Up until now I’ve only traded stocks and options, but I’m interested in broading my horizons to new types of investments. I’m just wondering if I need a new account, or if I can do this with my current brokerage.

is silver a good investment? such as silver bars and silver rounds?