A single 1.4GHz G4 would rock.
http://sonnettech.com/product/encore_stg4.html
2 GB of RAM would rock.
http://www.crucial.com/store/listpa...+Mac+G4+(350,+400,+450,+and+500MHz)&submit=Go
And, for the big graphics upgrade, nothing beats the Radeon 9800:
http://eshop.macsales.com/Catalog_Item.cfm?ID=6048&Item=ATI100435050
Get rid of that crappy CD or DVD-ROM in the computer and slap in the newest Pioneer DVD-R/W drive for right around $100... Mac OS X 10.3.3 and above have native support for this drive, so it will be bootable and recognized by all the iApps and stuff, just like as if Apple had installed it themselves:
http://eshop.macsales.com/Catalog_Item.cfm?ID=6483&Item=PIODVR107
Also, your machine only has a 66MHz ATA bus speed. Get an ATA/133 PCI card, and you can put 6 hard drives in your machine if you're careful (if you're really serious about hotrodding your G4, check out the RAID version of the card... or hell, even add SATA to your G4):
ATA/133:
http://sonnettech.com/product/tempo_ata133.html
ATA/133 RAID:
http://sonnettech.com/product/tempo_raid133.html
SATA:
http://sonnettech.com/product/tempo_serial_ata.html
And, get rid of that slow 5400rpm drive that Apple gave you. Put in something with some space, some angular speed and a huge cache for increased performance (hell, add two and RAID 'em!):
http://harddrive.com/search.do?searchItemNo=HI7+WDC+WD1200JB
I recently moved my drives off of the internal ATA/33 bus and onto a Sonnet Tempo ATA/100 card and I can tell a difference. Moving files about between 3 hard drives is lightning-quick. I get close to 50MB/sec transfer rate, which ain't too bad.
Good luck! If you've got the money to spend on that machine, go for it. With those upgrades, you can get a few more years of use out of your machine...
(By the way... I've purchased multiple items from every company I listed above and have had nothing but great prices, shipping and support. These are companies I trust and will continue to purchase from.)