I bought a 512mb stick of PC133 RAM for my sisters G4 a few days ago, and it arrived today. When I took out one of the 128s (previously there was 128, 128, 256, and 256 installed, all PC133) and popped in the 512. I checked About This Mac, and it only revealed 640mb, the 128 and the 256s, according to apple system profiler. I've tried the 512mb stick on its own in there, in all the slots, but every time I press the power button the G4 emits a very loud beep. IIRC, that beep means it can't find any RAM...
I then tried it in the Yikes! G4, and it works fine, but it only shows up as 256mb. I guess the biggest cards the Yikes! G4s can use is 256mb.
Any ideas? I'd really liks to have this card working in the sawtooth. I have tried it with no cards but the video card, with just one each of the other RAM cards, and with all the other RAM cards in difference slots.
Thanks! 
Yikes and Sawtooth G4s (as well as Beige and Blue & White G3s) will not accept 512MB sticks. They'll max out at 1024MB RAM using 4x256MB low-density sticks (Beiges max out at 768MB using 3x256MB).
Low-density RAM has chips on both sides of the stick, not just on one.
If you do try to use high-density 256's or any 512's, you'll find, as you have experienced, that the Mac will do one of three things: not boot up at all; boot up, but not see the stick; or boot up, but only see half the stick.
If you have any later G4s (Gigabit, Digital Audio, Quicksilver, or Mirrored Drive Door), you can use the 512MB stick in those. They'll max out at 1.5GB RAM using 3x512MB.
I think I may doubt you on that. I had a Sawtooth with 2x High Density 512MB sticks and 1x 256MB stick, and yes it was a sawtooth. Never had a problem.
According to Apple, the G4 AGP, Sawtooth, can take up to 2gb RAM. Which is 4x 512mb cards.
EDIT - Here's the link from Apple's site - link!
Also notice the note: "5. Although 2 GB can be physically installed, a maximum of 1.5 GB is recognized by Mac OS 9. Mac OS X will see the entire 2 GB."
So what's the brand of the memory and it's chips' brand and chip numbers and layout, etc.? You can find some SDRAM specs here:
http://www.chipmunk.nl/DRAM/ChipManufacturers.htm
The difference between the Sawtooth and the Yikes! is fairly profound--the Sawtooth being a real "sea change" from the architecture of the Yikes! which was basically a B&W without Apple's creepy G4 block--but since it works halfway in the Yikes! and not the Sawtooth at all makes me wonder if the RAM is actually Mac incompatible. It would be interesting to boot into OS 9 on the Yikes! with the RAM installed and see what DIMM First Aid says about the stick.
I've got a number of SDRAM sticks that work great in my Sawtooth G4 but refuse to be seen in my B&W, as well as the reverse case. In theory, all these sticks should just work in both machines but it just ain't so. I can't tell whether any given stick will work in either situation without actually trying it in the target machine (word target particularly apropos, I've felt the desire to blast the %^$#&%$& things numerous times.)
FYI, I've got some 512MB sticks I can swap back and forth between my QS and my Sawtooth, so they definitely can work.
My advice - buy from a vendor who will support their RAM in the target machine, examples of such long-established Mac-friendly vendors include OWC*, DMS, ramjet and smalldog.
dan k
* my fave
OS X is "picky" about RAM quality, while classic OS is not. Some RAM that works in OS 9 will not work when you upgrade to OS X. A possible "fix" is to run (in OS 9) DIMM First Aid http://www.versiontracker.com/dyn/moreinfo/macos/10415
Cheers, Tom
Mac Troubleshooting, Maintenance & Tips
http://www.geocities.com/texas_macman/MacTroubleshoot.html
Well, I tried booting into OS 9 to try out DIMM First Aid, and I think that killed it...(?)
It was booted into OS X, I opened sys prefs, chose startup disk, chose OS 9.2.2, and hit reboot. It chimed, then stayed at the screen with the blinking '?'. I tried booting off an OS 9 CD, and it doesn't see the HDD _at all_. I tried booting off a panther CD, but it just sits at the grey apple with the spinning thing forever.
I have tried with just one stick of known working RAM, as well as doing all the resets. It refuses to boot off the HDD...
Well, I left the Yikes! unplugged for a few hours and then tried starting it; it booted back up just fine with its original RAM, and it works great now. It really is too bad I can't use the 512mb stick in the sawtooth...
That link that Texas Mac Man provided above to versiontracker's DIMM First Aid download page is really worth checking out. Unlike the Mactcp DIMM First Aid download website, it's got a very long attempted explanation by Glenn Anderson--DIMM First Aid's designer--about RAM speeds and compatibility with Apple computers.
Unfortunately, I got about halfway through before I was too confused to continue reading. It's all a lot more complex than I had thought. I had thought that for the B&W and the Yikes! the optimal speed was simply 8ns PC100-2-2-2 (CL2). Now I see there's a lot more variables in the mix. How does one decipher all those numbers?