Starting with $500 gives some flexibility in how you can trade; $100 doesn't. If you want to day trade forex, start with at least $500. No matter what balance you start with, limit risk to 1% of your account balance on each trade. Alter the above scenarios to help determine what your position size should be based on the stop loss level you use and what type of lot (micro, mini or standard) you're trading.
Trading in the euro has grown considerably since the currency's creation in January 1999, and how long the foreign exchange market will remain dollar-centered is open to debate. Until recently, trading the euro versus a non-European currency ZZZ would have usually involved two trades: EURUSD and USDZZZ. The exception to this is EURJPY, which is an established traded currency pair in the interbank spot market.
The need to exchange currencies is the primary reason why the forex market is the largest, most liquid financial market in the world. It dwarfs other markets in size, even the stock market, with an average traded value of around U.S. $2,000 billion per day. (The total volume changes all the time, but as of August 2012, the Bank for International Settlements (BIS) reported that the forex market traded in excess of U.S. $4.9 trillion per day.)