FastMac gave me great service and it's humming along nicely here. The biggest advantage of FastMac is zero downtime, they only require you to send them a processor card which for me (in the UK) was the closer. I was able to buy a cheap 400Mhz card on eBay and send it to the US without losing any use. They are cheaper as well...............
The FastMac upgrade does require you to do the installation yourself of course, so if you're not comfortable with it go for Daystar.
Reviews can be found <A HREF="http://forums.xlr8yourmac.com/cpureview.lasso>here</A> by entering " Powerbook G3 2000 Rage128" and "other" in the manufacturer.
I find performance in Altivec apps to be massively better, my iTunes encoding doubled for example. In general use you won't notice a massive difference - most operations are dependant on processor SPEED and as 550Mhz is the fastest G4 that will fit it only gives a 10% overall speed increase. For all Altivec apps (which includes the Finder these days) you will see the difference, in fact I was able to get better results in xBench than a standard G4 PowerMac 800Mhz after all my upgrades.
Heat does increase somewhat, you've probably never heard the fan on your 400, you will with the G4 - although I must say that it's probably a bit cooler than my 500 G3 was, as I get less fan action than I did before even with 1GB RAM, etc.
I don't regret it at all but it may not be the right time now, with Apple committed to Intel for the future it may be better to save for a new 'Book. You'll still be stuck with your ancient graphics card and older technology screen so unless you REALLY love that Mac I'd say no at this point.
