Know your G5. There various models of G5. In each generation there are at least two motherboard layouts except PCIe model I think. What you can do depends on what you have.
SSD: All the PowerMac G5 support SATA I. You can use SATA SSD. As you are limited to SATA I speeds I wouldn't bother with the fastest model as they are all faster than you can fully utilize. Something like a Crucial BX500 is fine. If this is the last generation G5 with PCIe slots. There are some SATA II and SATA III (SATA or eSATA) cards available which are compatible. They'll most likely be used. Linked are some examples. There are likely a lot more compatible models. You'll just need to make sure they are compatible with Mac OS 10.5.8 or earlier and bootable.
https://www.sonnettech.com/product/legacyproducts/temposataiii6gbpcie.html
https://www.sonnettech.com/product/legacyproducts/tempo_sata_e4i.html
I'd probably just use the SATA I ports. Anyways, you'll also need proper 2.5" to 3.5" bay adapters. Not any old adapter will do. The mounting holes and ports need to line up correctly to simulate a 3.5" drive.
https://www.amazon.com/Fenlink-Internal-Hard-Drive-Converter/dp/B01ELRRKW8/
CPU: Depends on the exact year and even model of that year G5 you have. Some can't even swap the same CPU from the same model without Apple's Diagnostic disc for recalibrating. Some have a locked bus speed. So, while you can put in a faster CPU from the same model. It will just run at the same speed as the CPU you removed. Then other models allow simply switching out CPU from the same model year for different speeds. Any CPU will be a used pull from another G5.
RAM: The RAM you need will depend on which model of G5 you have. As it can be DDR or DDR2 and four slots to eight slots. Once you know what you need. Just get used RAM off eBay. It's really cheap. New DDR/DDR2 is ridiculously pricey or some questionable knock off brand.