Tiger IMO, has outperformed Panther on any machine I ever ran it on. Tiger was and still is an awesome OS. A 400MHz G4 is fine for Tiger. However even a modest 32MB Radeon will make a vast improvement in the GUI responsiveness of the machine. If you want to go all out you can:

Install a 1.8GHz G4

1GB PC100 / PC133 SDRAM

SATA / PATA Controller Card

Radeon 8500 or better GPU

Modern HD

Be prepared to spend some bucks though.

I'd rather drop that cash into a DP system which would run faster, have better upgrade possibilities and provide for a lengthened system life time.

Although I could never step down to a 400MHz box again, I can certainly understand for someone else who just now got a G4.

Congrat's on the machine, may it serve you well. 8)