If you want to place it just above the Quick Reply box, then edit topic.asp and find this code :
if strShowQuickReply = "1" and strDBNTUserName <> "" and ((Cat_Status = 1) and (Forum_Status = 1) and (Topic_Status = 1)) and ArchiveView = "" then
call QuickReply()
Directly above the IF you add in your banner code, it might be easier to do something like this :
%>
<!-- banner code goes here -->
<%
if strShowQuickReply = "1" and strDBNTUserName <> "" and ((Cat_Status = 1) and (Forum_Status = 1) and (Topic_Status = 1)) and ArchiveView = "" then
call QuickReply()