The resource monitor tool there is in Windows98/ME do for me.
I am not familiar with XP but I don't think such a tool exists in XP.
Anyway you can possibly measure the precise memory use of Reaper with Memload under XP :
http://www.free-pc.hu/pub/win9x/rendszer/memload.zip
If doing nothing else with Reaper than toggling between the tabs leads to an ever increasing use of the memory by Reaper, then it should indicate that the problem also exist with XP.