>I think v3 cards not working in v2 slots also won't work in v1 slots, and that v3 cards working in v2 slots not necessarily work in v1 slots.

Seems probable.

So the G5 likely tops out at the 6000 series, whereas the X5000 supports the 7000 series, most of the R5, 7, and 9 cards, and possibly some higher cards.