It just happens that I was reading some reference material tonight and ran across the answers to your questions.
Once multiple currencies have been enabled for your orgs by salesforce, you'll need to set up a "corporate currency" for your org that reflects the currency it does business in. You can then create a list to maintain of multiple currencies in which the company also does business in. Every user must also have their 'personal currency' configured for them (they can override it when creating new records).
As an aside, an opportunity's currency cannot be modified once it's been created using the default currency. You'll need to remove all records first before you can modify the opp's currency.
Next, the exchange rate is something that your org supplies based on readily retrievable data as the exchange rates vary daily. Since its something you can change, I doubt that SF will present you with a default value. At least that's not indicated from what I just read.
The difference between using Standard Currency Exchange Rates and Dated Exchange Rates is that the former has no history associated with it. However, when your org makes changes to a Dated Exchange Rate, the history is preserved for tracking. This is especially helpful when exchange rates fluctuate significantly. When a change is made to the Standard Exchange Rate, there's no history logged of what the previous rate was. This change affects all opportunities, forecasts and other amounts that use the current conversion rate; something which can significantly affect profit projections based on opportunities currently in the system. This is likely of more importance to large, multi-national companies than to small businesses.
Specifically, dated exchange rates are used in conjunction with opportunities, opportunity products, opportunity product schedules, campaign opportunity fields and reports related to those objects/fields.
A few caveats to be aware of:
When Advance Currency Mgmt is enabled, VF input and output fields cannot display currency fields.
If enabled, you also cannot create roll-up summary fields that that calculate currency on the opportunity object.
Enabling multi-currency may have some unintended consequences regarding SOQL queries and aggregate functions. The material I was reading suggests you check the documentation for more about the implications of enabling multi-currency.