I use it on my iBook 600 with 384 MB of ram. I run Win 95, 98, 2000, NT, and XP as well as Mandrake Linux. I am a network admin, and it helps to be able to boot to any OS that my customer may be using. I have to say that the performance in X is not bad if you do a few things.
1st) Keep the drive format Fat 16 this will make it much faster than Fat 32, or NTFS.
2) Defrag, Defrag, Defrag, this makes a huge difference in the Virtual Machine.
3) Give it plenty of memory, but not the maximum that you can.
EX: I have 384, I can allot 212, but I only give them 160. This will give OS X more memory to manage the Mac efficiently.