6GB of RAM should be more than enough. We have been playing with 8GB and i3/i5/i7 CPUs. Your bottleneck could be a number of things, but CPU would be my guess if you have that much RAM. A lot of CPUs were not even compatible with XP Mode as they did not support virtualization. Then, MS put out a patch for that allowing XP Mode, even if the CPU didn't support vitualization.
I did raise the default RAM usage in the Virtual PC to 2GB and everything has been running smoothly. The only thing I have noticed so far is that I have to reclaim a USB dongle that I was using every time I start up Flexi.
I have played with Virtual PC (XP Mode) on many systems (Windows 7/Vista/XP)and it has been pretty good as long as the PC is adequately equipped. I am not certain why the software "sucks" on your particular PC. I am just stating, that in my experience, in most cases, it does what it is supposed to do.