Depending on which G5 you have, you can run anywhere from 4GB to 16GB
(Considering they never made a 2.1, I'm not sure what you have)
If you have at least a Dual 2, and you absolutely need to use it, and if you ever wanna do anything remotely intensive:
then max out your RAM... just look for it on Craigslist and find it cheap.... not worth spending a lot of money on.... I can buy complete DP 2.0's all day for $100
If it's the Quad 2.5, it's a no brain'er - max it out for sure.... even though it's old, and stuck on 10.5.8, it is a very capable machine for what it can run (software version limitations)
For reference:
1.6 (PCI) 333 MHz PC2700 DDR 4slot 4.0 GB
1.8 (PCI-X) 400 MHz PC3200 DDR 8slot 8.0 GB
2.0 DP (PCI-X) 400 MHz PC3200 DDR 8slot 8.0 GB
1.8 DP (PCI-X) 400 MHz PC3200 DDR 8slot 8.0 GB
1.8 DP (PCI) 400 MHz PC3200 DDR 4slot 4.0 GB
2.0 DP (PCI-X 2) 400 MHz PC3200 DDR 8slot 8.0 GB
2.5 DP (PCI-X) 400 MHz PC3200 DDR 8slot 8.0 GB
1.8 (PCI) 400 MHz PC3200 DDR 4slot 4.0 GB
2.0 DP (PCI) 400 MHz PC3200 DDR 4slot 4.0 GB
2.3 DP (PCI-X) 400 MHz PC3200 DDR 8slot 8.0 GB
2.7 DP (PCI-X) 400 MHz PC3200 DDR 8slot 8.0 GB
Dual Core (2.0) 533 MHz PC2-4200 DDR2 8slot 16.0 GB
Dual Core (2.3) 533 MHz PC2-4200 DDR2 8slot 16.0 GB
"Quad Core" (2.5) 533 MHz PC2-4200 DDR2 8slot 16.0 GB