First suggestion: be specific.
Since your sig says iMac G5, and you say you have 2 memory sockets, then by inference you have a 1.6, 1.8 or 2.0 GHz iMac G5 from before the iSight model came out. Therefore, you are looking at DDR PC3200 (400 MHz) 184 pin DIMM RAM
The iMac G5 is THE most finicky desktop Mac when it comes to RAM compatibility. If the SPD settings are not correct on the RAM module, it will fail. Therefore I don't recommend trying PC generic RAM in this machine, stick to RAM that is tested and guaranteed compatible with your specific machine.
You can install a 1 gb beside an existing 512. When you need to move up to the machine's maximum of 2 Gb, remove the 512 and install another 1 Gb module.
Second suggestion: If asking "Where to buy" questions, how about telling us where you are? Freehold, Mars, that doesn't help.
If you are in the USA, the top three vendors cited here at MR are: Crucial.com, OWC (Macsales.com) and my recommendation:
Data Memory Systems.
You are looking at $88 - $95 per Gb.
Third suggestion: Type with conventional spelling and punctuation. More people will be willing to help you if you make it easier to read. Save the other stuff for your cell phone.
Thanks
Trevor
CanadaRAM.com