Optimum file buffer read size?

I am writing an application which needs to read fairly large files. I have always wondered what's the optimum size for the read buffer on a modern Windows XP computer. I googled and found many examples which had 1024 as the optimum size.

Here is a snippet of what I mean:

long pointer = 0;
buffer = new byte[1024]; // What's a good size here ?
while (pointer < input.Length)
{
    pointer += input.Read(buffer, 0, buffer.Length);
}

My application is fairly simple, so I am not looking to write any benchmarking code, but would like to know what sizes are common?