Which way have you mounted your XP 90, I have the tubes at the top on my Asus P4C800-E. The HS will fit either way. The Thermalright's info is not very clear on how to mount it.
Mine is mounted so that the heatpipes face to the I/O plate and the extention that hangs over past the CPU is over my RAM. I found this was the easiest to mount, also make sure you secure the corners without the heatpipes first as said in the instructions manuel.