Adding RAM to the PowerMac G4

I apologize if this has already been answered, I quickly did a search but didn't find anything to solve my issue.

My Mac knowledge really stops at the PPC G5, so I don't know much about the G4t & a small prodution I am working for said "Hey, can we run Final Cut Pro 2 on here?"

So first, I think that I have a good enough video card - the ATY, RV250 (ATI Radeon 9000 Pro video card.)

Secondly, the RAM, I have no idea about the RAM this can take, or what type. Here's the info I have on it:
DIMM3/J20:

Size: Empty
Type: Empty
Speed: Empty

DIMM2/J23:

Size: 512 MB
Type: DDR SDRAM
Speed: PC2600U-25330

DIMM1/J22:

Size: 512 MB
Type: DDR SDRAM
Speed: PC2600U-25330

DIMM0/J21:

Size: 256 MB
Type: DDR SDRAM
Speed: PC2600U-25330

I'd assume I could get up to 2GB at least?

Also here's my machiene info:
Hardware Overview:

Machine Model: Power Mac G4
CPU Type: PowerPC G4 (2.1)
Number Of CPUs: 2
CPU Speed: 1 GHz
L2 Cache (per CPU): 256 KB
L3 Cache (per CPU): 1 MB
Memory: 1.25 GB
Bus Speed: 167 MHz
Boot ROM Version: 4.4.6f2
Serial Number: ########
Sales Order Number: #########

Lastly, since this Mac is still at 10.3.9, does anyone know if I can upgrade to 10.4? or 10.5? (if that would work...)

I'd appreciate any help. Thanks in advance.