Just as a first suggestion: Try to avoid machines with the "smallest" Mobile graphics accelerator. I have a Nvidia GeForce Go 6150 in my laptop and there isn't even a new driver available! For the 7000 and 8000 series there are drivers, but none for the 6150.
I can run Orbiter (AMD Turion 64 X2 TL-50 1.6 GHz, 2GB RAM, Vista Home Premium 32bit), but only at framerates up to 17 or something like that.
Craziliy enough, the stationary Vista PC next door (Core 2 Duo 1,98 GHz, ATI Radeon X1150, 2GB RAM) runs Orbiter with framerates as low as 4 when I set the resolution to 1440x1440 (I have a widescreen TFT monitor). The XP PC (P4 3GHz, 512MB RAM, ATI Radeon 9800XL, WinXP SP3) runs Orbiter with 40 FPS at 1600x1200 !!!
Maybe you want to make a multiboot system with the default Vista SP1 and XP SP3.