Shuttle doesn't recommend upgrading to a different motherboard as it could cause hardware issues.
However, there is one model that has an upgrade option and this is the XPC Barebone SN68PTG6 Deluxe which you could upgrade with the motherboard FN78.
The maximum size of a card that fits in the G-Series is 206 mm (L) × 98 mm (H) × 16 mm (D). But due to the different layout designs of each model and VGA card, we strongly suggest to check our VGA support list for each model or test before purchasing the VGA card.
This is a known issue of NVIDIA chipset, if you want to install windows OS with SATA ODD, please change the SATA setting form “IDE” to “AHCI” in BIOS → “Integrated Peripherals” → “MCP Storage Config” → “SATA Operation Mode”, then install windows.