As a Snow Leopard and VM user myself I would make the following suggestions.
1 - Max out your hardware
Upgrade your RAM and HDD as these two are the real killers when it comes to performance for VMs. Don't even try running a VM when stored on external drives unless you've got the latest Thunderbolt interface...
My old 2008 edition Macbook for example is advertised as 4GB max but can actually take 6GB. If you hardware is out of warranty and you have that option, consider it.
Upgrade to SSD. This alone will make a huge difference. With SSD prices falling this a must for anyone using VMs alot.
Another alternative I've considered recently is replacing my macbook's DVD drive with a second SSD or 7200RPM large capacity HDD. This, together with a large primary drive could possibly be setup as a striped RAID volume to improve disk guest performance.
2 Configure your VMs correctly
Don't give more resources to your VMs than necessary. Especially if you need to run more than one at a time. If you are running linux, consider running it without the X server (GUI) as it is a CPU/RAM hog.
Your VMs disk should be pre-allocated for best performance.
Disable any and all unnecessary services, especially disk related.
3 OSX setup
Your OSX minimal idea is not a bad way to go. Of course it goes without saying that you don't install Antivirus software. If you absolutely must, configure it so that it doesn't impact the system. I have mine with real time scanning disabled.
OSX Spotlight and similar programs that monitor the filesystem could also be disabled.
Another issue that you need to be careful of is OSX filesystem fragmentation. Make sure that the disk where the VM is stored has a lot of space free so that the files in OSX representing the virtual HDDs are contiguous. This is especially problematic if you haven't pre-allocated your disks and/or you work with snapshots.
4 Use alternatives to VirtualBox
I realise that VirtualBox is attractive because it is free, but I don't know how well it performs versus a non-free product like VMWare Fusion or Parallels. I've seen articles that argue that they perform better.