People with B&W towers are in a predicament. The 1GHz upgrade costs $300-400 depending on who you buy it from.
For around $300-500 (from a reseller/ebay) you could buy a Powermac G4 "Sawtooth" 400-450MHz with an AGP slot, faster bus etc. The processor is slower but it can accept high-performance AGP video cards. Plus, it can accept CPU upgrades up to 1.7 GHz or dual 1.33GHz currently. With a video card/hard disk/CPU upgrade you can build a very fast Mac. This is what I'm doing with my G4 "Digital Audio".
You could also get a Mac mini. It has a slow laptop hard drive and not a lot of current upgrade potential. By the time you add RAM and Airport etc. expect to pay $600-700+. But it is a LOT of computer for the money, especially when you compare it's performance with your upgraded G3 (MUCH faster). It will be quite speedy in everything except games.
The G4 tower will offer you the highest performance POTENTIAL but the greatest total investment. The mini is fast and the most bang for the buck but the greatest initial investment. The G3 upgrade is the cheapest but the lowest performance by a good margin. A 1GHz G3 is no match for a 1GHz G4, although a lot of the difference is due to the motherboard design.