Any pointers to good guides ? (Ubuntu, VirtualBox preferred. 8GB quad-core AMD PhenomII.)
It's all pretty much straight forward, but I found it most stable when I: use the darwin_snow.iso provided as your boot device (it emulates the bootloader used by Macs), use all SCSI devices, and in the config file, set guestOS = "darwin10-64"
The machine I'm running it on now is a i5 750 with 12 gb of mem; I have a single core and 2gb of mem dedicated to the VM. Feel free to email me if you need any help, I spent quite awhile playing with various methods of virtualizing Snow Leopard, so I can probably answer a majority of questions about issues that arise.
edit: As far as VirtualBox, I'm not aware of anyone getting 10.6.2 to run stable in it. I believe 10.4 will run okay. You may want to just use VMWare player, though. Also, you will have issues with the AMD processor, Apple was 'thoughtful' enough to include code to ensure Mac OS will only run on Intel processors. There are several distributions of 10.5 that have been modified to run on AMD processors, but I haven't really played with them - I think google can assist there, though.
I picked an AMD CPU partly because a sort-of-server box should have ECC RAM [1], which AMD does support 'for free' (and also toss in sufficient graphics in the chipset); it seems that with Intel CPUs one needs to go to very expensive MBs or the Xeon line for that.
[1] D.J.Bernstein, for one, explains why: http://cr.yp.to/hardware/ecc.html , http://cr.yp.to/hardware/build-20090123.html
P.S. Unfortunately, D.J.Bernstein refers to RAID-5 as "an easy-to-use automatic backup mechanism" (if a drive dies); he either didn't get the "RAID is not backup" memo, or does not believe it (until the day he accidentally wipes data ;-).