VB.Net Text File Splitter

Private Sub btnGo_Click(sender As Object, e As EventArgs) Handles btnGo.Click
		btnGo.Enabled = False
		lstOutput.Items.Clear()
		Dim sb As New StringBuilder(300000)
		Dim Suffix As Integer = 1
		Dim MaxLines As Integer = CInt(txtLineNum.Text)
		Dim outName As String
		Dim i As Integer = 0

		outName = ShowProgress(Suffix)
		For Each line As String In File.ReadLines(txtFilename.Text)
			sb.AppendLine(line)
			i += 1
			If i = MaxLines Then
				i = 0
				Using op As New StreamWriter(outName, False)
					op.Write(sb.ToString())
					op.Flush()
					op.Close()
				End Using
				Suffix += 1
				sb.Length = 0
				outName = ShowProgress(Suffix)
			End If
		Next
		If sb.Length > 0 Then
			Using op As New StreamWriter(outName, False)
				op.Write(sb.ToString())
				op.Flush()
				op.Close()
			End Using
		End If

		sb = Nothing
		btnGo.Enabled = True
		MsgBox("Done", MsgBoxStyle.Exclamation)
	End Sub
	Private Function ShowProgress(ByVal fileNum As Integer) As String
		Dim oName As String = txtFilename.Text & fileNum.ToString("000") & ".txt"
		lstOutput.Items.Add(oName)
		lstOutput.Refresh()
		Application.DoEvents()
		Return oName
	End Function