560 Ti won't work with G5s as Apple used a different firmware back in the PowerPC days. Best GPU available for G5s is a 512 MB 7800GTX.
You can get 560s to work with early Mac Pros (1,1 and 2,1) so that would be an option, otherwise purchase a 7800 GTX.
could i flash a 7900 with a 7800 rom
Nope, sorry. If it could have been done, it would have been done.
Are there any 7990 roms available for flashing?
There has to be an open firmware rom witten and ready for flashing.
could i flash a 7900 with a 7800 rom
As far as G5 towers go, the best you can get, in no particular order, are the AGP Gainward Bliss GeForce 7800 512MB (half that memory usable) and a top-level binned flash ATI Radeon Fire GL X3 (my OpenMark score is a little over 22K), and the Radeon X1900 for PCIe models.
-7800GTX 512mb
For the OP: I've had difficulties tracking down the ROM, might want to find it before purchasing.