Index funds can be invested in via a standard brokerage account such as scottrade, etrade, zecco being the cheapest, i think....etc. Or you can use your 401k, if you have one. 

Few things to know:

ETFs are cheaper than index funds, in terms of fees
If you plan on trading often, fees can become substantial 
In terms of rate of return, it depends on the fund that you're planning on investing in

IMO, the time to invest in Gold has passed, since the prices have increased within the past year, and you really wouldn't be getting the biggest bang for the buck for the short term. But Gold isn't the only precious metal that you can invest in, there are other commodities that you can invest in such as oil (in the simplest of terms), but that's where it starts to get complex. Check Yahoo finance for GLD, and analyze its one year trend.

In the end, it all comes down to what suits you best in terms of risk, I know I'm not being of much help but there's no escaping your due diligence. The one thing I would suggest against is investing in something that you don't understand (per warren b). 

Good luck!