Out of Memory using TextWriter Stream with HttpWeb

Question

Thanks for your great suggestions to an OOM (out of memory) problem I'm seeing in code intended to stream files for web services. [I hope it is OK to start another thread which provides a bit more detail.] From the suggestions, I shrunk the buffer size used to read from the file, and it looks like memory consumption is better, but I'm still seeing an OOM problem, and I'm seeing this problem with files sizes as small as 5MB. I potentially want to deal with files ten times larger.

My problem seems now to be with the use of TextWriter.

I create a request as follows [with a few edits to shrink the code]:

HttpWebRequest oRequest = (HttpWebRequest)WebRequest.Create(new Uri(strURL));
oRequest.Method = httpMethod;
oRequest.ContentType = "application/atom+xml";
oRequest.Headers["Authorization"] = getAuthHeader();
oRequest.ContentLength = strHead.Length + strTail.Length + longContentSize;
oRequest.SendChunked = true;

using (TextWriter tw = new StreamWriter(oRequest.GetRequestStream()))
{
    tw.Write(strHead);
    using (FileStream fileStream = new FileStream(strPath, FileMode.Open, 
           FileAccess.Read, System.IO.FileShare.ReadWrite))
    {
        StreamEncode(fileStream, tw);
    }
    tw.Write(strTail);
}
.....

Which calls into the routine:

public void StreamEncode(FileStream inputStream, TextWriter tw)
{
    // For Base64 there are 4 bytes output for every 3 bytes of input
    byte[] base64Block = new byte[9000];
    int bytesRead = 0;
    string base64String = null;

    do
    {
        // read one block from the input stream
        bytesRead = inputStream.Read(base64Block, 0, base64Block.Length);

        // encode the base64 string
        base64String = Convert.ToBase64String(base64Block, 0, bytesRead);

        // write the string
        tw.Write(base64String);


    } while (bytesRead !=0 );

}

Should I use something other than TextWriter because of the potential large content? It seems very convenient for being able to create the whole payload of the request.

Is this totally the wrong approach? I want to be able to support very large files.