Faster way of binding multiple rows of data than rbind?

Hi all,

I have a vector that contains the row numbers of data taken from several 
filtering operations performed on a large data frame (20,000rows x 500cols).

In order to output this subset of data, I've been looping through the vector 
containing the row numbers (keepRows).

output <- data.frame(row.names = rownames(bigMatrix))

for(i in keepRows)
{
	output <- rbind(output, bigMatrix[i, ])
}


As you may guess, doing all of these rbinds takes a LOT of time, so I'm 
wondering if there's a workaround where I can maybe use an intermediate 
matrix-like object to store the loop output, and then coerce it back to a 
data frame after the loop is complete??

Thanks in advance,
Ken