Just installed a Sunon HA40201V4 MagLev (12.8dba) fan in my Ultra.
Verdict: Not a massive difference, but it's an improvement. It whined pretty badly when mounted too near to the rear panel, so I had to tweak the distance with silicone supports.
It's more powerful than the old fan, so it should work nicely on a reduced voltage. I'm thinking of trying this controller, because with a bit of hacking it can probably be mounted in an option port:
http://bit.ly/ahfPjY
The only thing that worries me is that some other PC fan speed controllers take power from a molex plug, rather than the motherboard header, because otherwise the header risks being "damaged". I don't know enough about electronics to know why this would be. Is anyone here familiar with this issue through PC building/modding, etc.? Thanks.