The G5 does not have an Intel processor it has an IBM Power PC processor.
An "Intel-based iMac G5" would be an oxymoron. The "G5" part of it's name refers to having a PowerPC processor.
The iMac G5 used a G5 processor - hence the name. This was the last of the PPC processors that Apple used before switching to the Intel processors. It followed on from the G3 and G4 processors found in earlier iMacs and iBooks.
CrossOver requires a Mac with an Intel Core Duo processor to replicate a Windows type computer. It will not work on a G5 iMac.
Sims 3 will not work with a G4 iMac. The game requires a recent Mac with an Intel processor.
It is not possible to replace the processor in a G5 iMac. You could replace the entire logic board for one with a slightly faster G5 but it will be a lot of work for little gain. The newer Intel Macs are completely different systems and the parts are not interchangeable with older Macs.
Mac OS X 10.6 (Snow Leopard) only works with Macs with an Intel processor. The last version that will work with a G5 Mac is 10.5 (Leopard).
The G5 came with the OS 9 operating system and had a Power PC processor. It was the last edition of Mac OS. The iMac switched to the OSx operating system with the next iMac. There was an education only model G5 that did ship with OSx Panther.
As Sims 3 requires a Mac with an Intel processor it will not run on a Mac with a G5 processor.
The current Mac Pro uses the Intel Xeon Quad Core processor. Earlier incarnations would have had a G5 processor.
To upgrade beyond Mac OS X 10.4 (Tiger) you will need to purchase Mac OS X 10.5 (Leopard). If the iMac has an Intel processor you can jump straight to Mac OS X 10.6 (Snow Leopard) but if it has the older G4 or G5 processor then 10.5 will be as far as it can go.
There was no Mac Pro G5. The Power Mac G5 was discontinued in 2006 and replaced by the Mac Pro with an Intel Xeon 5100 series "Woodcrest" processor.
If it has an Intel or G5 processor, then yes. If it has a G3 processor, then no. If it has a G4 processor, then it depends on the processor speed.