Well it makes some difference whether your 20" is an intel iMac 20" or a G5 iMac 20"
Intel = Yes you can swap RAM
G5 = No, not a chance
If I was betting on it, with a 1.5 Gb configuration chances are you have the G5 iSight model with the 512 soldered-on and one DDR2-533 DIMM -- you'll be outta luck on that.
Thanks
Trevor
CanadaRAM.com