Based on today’s market value of gold (7/13/2011), the nisab amount in US dollars is $4682.95.
[Taken from http://www.e-nisab.com/]
One is obliged to pay sadaqat al-fitr if one owns nisab hirman al-zakat, i.e., the above amount in any form of wealth aside from outstanding debts and one’s basic personal needs (such as house of residence, clothing, furniture, personal vehicle, etc). [Shurunbulali, Ascent to Felicity]
And Allah knows best.