According to EveryMac, it's 1MB VRAM, non-upgradable, for the onboard video - room enough for two full 800x600 8-bit framebuffers. I'd expect the issue lies more with me using a VGA monitor + adapter and the Mac having no idea what modes are supported, but if it would just let me select the all-supported-modes list instead of the recommended, it wouldn't be an issue...