Opinions on db operating systems Wanted

I am interested in hearing anyones opinions on what OS they would choose to run a MySQl db and the reasons why, of course. I have a task to build a db server for a project that will be very busy if things work as the creative minds think that it will. I am running a FreeBSD box right now on Athlon MP, but this box will be taken as soon as it is replaced by this machine right now. This machine works fine under current load, but I am not making the decisions. I am just interested in figuring out the best choice now, instead of finding that I could have made a better choice down the road when we are getting pouded with queries.

We are thinking about dual Opteron procs. Is there a better OS to run a 64 bit Mysql Innodb db in anyones experience?

Thanks,
Bob