If anyone here is successfully running Linux Mint and PC-BSD on two dedicated hard disk drives, (no emulator or partitioning stuff), using Phenom II or Athlon II CPU, I'd like to ask your help to pick hardware!
The important question isn't the CPU but the everything else...
I prefer advice from a pathfinder (one that has been where I'm seeking to go), and CPU is as good a place to start as any.
I was an early adopter to dual-core (well, dual processor) x86 64-bit computing. I had a 64-bit SMP/NUMA Opteron 242 system up and working back when most 64-bit Linux distributions were still a dysfunctional mess. Since then I've worked with many kinds of multiple-core processors, mobile and desktop versions, Intel and AMD, running 32-bit and 64-bit Linux kernels.
I maintain that the board and the peripherals are more important than the processor sitting in it. Your processor is an x86_64 compatible like everything else on the market, and ordinary 32-bit or 64-bit kernels will load on it. Whether it can do anything with it once loaded depends heavily on the system hardware and firmware; I got 64-bit Linux booting on my Opteron the very first try, the difficult bit was getting the disk controller drivers to work!
This is even frequently true for internal CPU features! Plenty of things like advanced CPU power-management modes, integrated thermal sensors, dual channel, integrated memory controllers, CPU chipset features, etc. only work if the manufacturer bothers hooking up the right wires. Frequently they don't, or just a small subset, to reduce size or cost or just to stratify the market. Ever see a laptop without C-states? Dell's sold a few.
IOW: Knowing your CPU doesn't even begin to tell you what you need to know about your system. Having a Phenom or Phenom II means you've probably got hypervisor support, if that matters to you.
So let's say you wanted to build PC having three dedicated hard disk drives (one for XP Pro, one for PC-BSD 8.1, and on for Linux Mint 7). I'm hearing on other forums that either CPU, motherboard/chipset, graphic card, etc., must support all three operating systems...or Linux Mint, XP Pro, & PC-BSD must have drivers for each component used in the PC. Not sure which is correct. Anyway, how do you determine which specific components will either run all three operating systems at optimal ability...or at least be compatible with all three?
I already answered that:
For the rest, there's no way around needing to have someone, somewhere, actually try it, so find a model you'd like, and google "linux mint motherboard-i-want".
You might have a harder time getting XP to work on it than Linux, though! It really doesn't want to install on modern computers, being very picky about disk drivers. You might be able to slipstream some modern drivers onto an xp-pro disk for it though.