The Quicksilver (and Digital Audio) is limited to 1.5GB since the PC133 memory bus only supports 3 sticks/slots and 1GB modules are not supported, so 3x512MB is your limited.
If you want 2GB, you will either have to get a Sawtooth, a Mystic/Gig Ethernet or go later and get an MDD. All three have 4 slots so support 4x512MB, for 2GB. 1GB sticks wouldn't be supported until the G5.
The only Power Mac Gx machine that can accept more than it's official limit of memory is the Early 2005 G5 models, which since based on the Xserve G5 supported 16GB of PC3200 with 8x 2GB sticks, rather than the official 8GB. The reason you don't see them often is because 2GB sticks of DDR1 are very hard to find.