First it's important to note that your minimum margin requirement (MMR) will vary depending on where you live. For example, I live in the US, where US regulations require the MMR for forex trades to be at least 2%. That allows me up to 50:1 leverage.
Therefore, I must set aside $20 as used margin for each 1k (AKA micro lot or 0.01 lot) position I open, or $200 for each 10k (AKA mini lot or 0.10 lot) position in USD/JPY. That means, if you set aside $1000 as used margin, you could open a 50k (AKA 5 mini lot or 0.50 lot) position in USD/JPY. However, that would leave you with no usable margin to maintain your position.
For this reason, even though I can use up to 50:1 leverage in the US, I try not to use more than 10:1 leverage. You may find this earlier discussion useful in explaining the reasons why: http://forums.babypips.com/newbie-island/66844-would-definitely-sell-position-right-3.html#post637676
As you expected, the exact MMR varies depending on the currency pair, and you can see these amounts in the MMR column of the Simple Dealing Rates window of the FXCM Trading Station.