Historical Currency Valuations

Dear WSN'ers,

I am looking for information on either a website or a good reference text that 
I could use to translate historical currency figures into a relatively modern 
dollar equivalency.  For example, my particular problem right now is to convert 
8 million rubles circa 1799 into a modern dollar amount.  However, I would like 
to get a good general reference source for future use.

Thanks in advance for any and all suggestions!

Jon Carlson