I've been using VirtualBox from Sun and using it with Ubuntu. I had only a couple of guest: WinXP and Win98, but they work just fine. Well, maybe not Win98, which runs slower then WinXP. I can't understand it why u haven't included VirtualBox in your review...
On the same pc but another hardisk, I have WinXP running VirtualPC with only Win98 (which runs faster than in VirtualBox) that I used mainly to run old games and apps which are not supported by new Windows OSes like WinXP.
Overall, I would use VirtualBox exclusively if it runs Win98 as fast as WinXP. It makes you think with an almost the same specs you specify -- 521 Mb RAM, 128 Mb video RAM, HD space, etc.. -- you'd expect Win98 to behave like a Ferrari, but instead you get a Model T.
