i'm not sure what other GPU's you can drop in there but here's the full run down of your system - source from
http://www.apple-history.com
CPU
CPU: PowerPC 750
CPU Speed: 300/350/400/450 MHz
FPU: integrated
Bus Speed: 100 MHz
Data Path: 64 bit
ROM: 1 MB ROM + 3 MB toolbox ROM loaded into RAM
RAM Type: PC100 DIMM
Minimum RAM Speed: 100 MHz
Onboard RAM: 0 MB
RAM slots: 4
Maximum RAM: 1 GB
Level 1 Cache: 32 kB data, 32 kB instruction
Level 2 Cache: 1 MB backside, 1:2
Expansion Slots: 3 64-bit 33 MHz PCI, 1 32-bit 66 MHz PCI (filled)
Video
Video Card/Chipset: ATI RAGE 128
VRAM: 16 MB
Max Resolution: all resolutions supported
Video Out: VGA
Storage
Hard Drive: 6-12 GB (up to 3 36 GB available BTO)
ATA Bus: Ultra ATA
Zip Drive: optional
Optical Drive: 32x CD-ROM, DVD/DVD-RAM available
Input/Output
USB: 2
Firewire: 2
Audio Out: stereo 16 bit mini
Audio In: stereo 16 bit mini
Speaker: mono
Networking
Modem: optional 56 kbps
Ethernet: 10/100Base-T
Miscellaneous
Codename: Yosemite
Gestalt ID: 406
Power: 200 Watts
Dimensions: 17" H x 8.9" W x 18.4" D
Weight: 28.7 lbs.
Minimum OS: 8.5.1
Maximum OS: 10.4.3
Introduced: January 1999
Terminated: September 1999
Although it shares the name of its predecessor, the "Blue" PowerMac G3 is an altogether different animal. Sporting an all new translucent "easy-open" case design (code named "El-Capitan"), the new G3 was the first Apple model to support FireWire, Apple's new high-speed serial standard. It was also the first professional model to include USB, although it also came with a "legacy" ADB port for backwards compatibility. In a controversial move, Apple chose not to include standard serial ports, a floppy drive, or on-board SCSI (Apple instead chose Ultra ATA). An internal Zip was available, however, as were SCSI expansion cards. The G3 was available in a number of configurations, starting at $1599, and rounding out near $5000 for the fully loaded server configuration. In late April, the "Blue" line was speed-bumped by 50 MHz, bringing the high-end model to 450 MHz.