> So 1 x 2GB RAM, which specs?

DDR2 SDRAM, DDR2-533 PC2-4200 or faster.
The 2.5 GiB maximum is outdated Wikipedia info, sorry. People have managed to get up to 4.5 GiB to work:

https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=13558&forum=3&start=7 (#8)
https://www.reddit.com/r/VintageApple/comments/n53nqn/so_the_isight_g5_imac_tops_out_at_25gb_does_it/

> Recommended SSD that can work on G5 old SATA quite well?

See the linked thread, and threads linked from there.

> GRUB - can it pick MOS and MacOS X choices?

Good question. I don't know, sorry.

>> I'm not sure about ExFAT for PPC Mac OS, but NTFS-3G exists for it.

> I wonder could MacOS X see NTFS partition

Yes, at least with NTFS-3G installed.