I can get the stanalone version of GPO to work, but none of the plugins will load in Cubase 4 (XP64). Also, in standalone mode, I can't go less than 256 sample buffer on my Fireface without cracles, is this normal?
256 samples, while not small, is not an unreasonable size buffer for soft synths. As for Cubase64, Kontakt Player2 is not a 64-bit plug-in, and the problems are likely due to how it handles the 32-bit/64-bit connection (and how Kontakt has been programmed). Rest assured you're not the only person experiencing difficulties running Native Instruments stuff in x64 land. The "simpler" situation of running the 32-bit standalone outside of Cubase's internal bit-adapting might explain your success there.