The folks at Thermalright emailed a user over at pcper.com's forums, and told him that the XP-120 will be a very tight fit on this board, but that the XP-90 WILL work and has been tested successfully with this board (the VNF4 Ultra). Incedentally, the user who was emailed had already bought the XP-120, and managed to fit it to the board (he had to "lean" one of the capacitors in order to do it, but the board has been running fine since, with no problems and quite cool. Let me know if you want a link to the thread over there about it.
If you are looking for good cooling, and low noise, you might look into the Coolermaster Hyper6. This is the cooler I have on my board, and it fits fine (although it is a tight fit). The stock fan IS probably going to be noisier than you would like, but this can be replaced, and you can NOT beat the cooling this HSF will provide. My system is overclocked to 300HTT (which puts my A64 3000+ at 2.7GHZ, and it is running at 22 degrees C at idle, and up to 30 degrees C under heavy load in prime95! If you want additional cooling, a second fan can be added to the rear of the heatsink, to help pull out the air that the first fan pushes into the heatsink. Also, because of the way this motherboard is designed (with the CPU slightly lower and to the center of where most boards have it, and with the RAM above it), it provides EXCELLENT airflow for this particular cooler -- it sucks air straight in from the front of the case, and pushes it straight out of the back fans/vent. It is almost as good as the new cases with the "wind tunnel" design. The one disadvantage to the Hyper6 is that it is HEAVY -- this thing is over 2 pounds because of all of the copper involved, but it clears all of the motherboard features with no problems, and it DOES provide the best air cooling that I have EVER seen.
So technically you CAN use the XP-120, you should be able to use the XP-90 without any issues, and you can also go with the Hyper6 if you so desire. Let us know what you choose, and how it worked for you.