If you look at the ads to the right of this post, you will see some places to get some ideas. The World Gold COuncil has a very helpful website, too.
More broadly, you can buy physical gold through a bullion dealer - suitable for a one-off, lump sum purchase.
You can buy shares in something like the Merrill Lynch Gold and General Fund (one of the best performing funds over the past few years) which will give you exposure to gold and other metals.
You could invest in something like RAB Capital or its Special Situations fund - both have exposure to minerals including gold and will probably have exposure to 'softs' like wheat. Look at someone like Jim Slater, who's been high-profile recently about commodities and Brazil, if you're prepared to trust an individual.
You could buy shares directly in companies that hold or mine gold or which explore for gold - Mercator Gold, Oxus Gold, Central China Goldfields - all of which have greater or lesser risk.
You could find a unit trust/investment trust that specialises in metals/commodities and which allows regular monthly investments of �50 - one with an ISA-wrapper might work. Try the AITC website.
Short answer - talk to an indepencent financial adviser and/or read the weekend money sections in the broadsheet newspapers.
Hope this helps.