I have a Dimension 4550 P4 2.4 GHz w/ 533 bus. It came with one stick of PC2700 memory. However, I wanted to add some PC2100 memory to it since the manual says that it is compatible and I have also read that the motherboard will take it. After I inserted the stick of PC2100, the computer doesn't start up and makes a beeping sound. But after I replace the memory with PC2700, it works fine. So then I thought I might have to change the BIOS setting for memory speed. However, the clock speed is fixed at 333Mhz and cannot be changed.

My question:
Does this computer actually take PC2100 memory?
If so, how do I get PC2100 memory to work?

Why go from PC2700 to PC2100? It's a downgrade.

The motherboard should automatically detect the type of memory and will set the speed accordingly in the BIOS. I don't know what BIOS version comes with the 4550. It may be that it is not user configurable. Not sure about this though.

It could be that the motherboard does not like the brand. I've seen this happen many times. What brand of PC2100 memory did you buy? It's always good to stay with the name brand memory like Corsair, Crucial, Mushkin, etc. Cheap, generic memory can sometimes cause problems. Motherboards can be very tempermental.

DH has the same machine, had the same problem- only all the doc said he had PC2100, when actually it was PC2700. When he bought extra 2100 it wouldn't work, and finally he got more 2700 and the machine was happy.

Yes, I know it's a downgrade, but since I had a bunch of PC2100 lying around, I thought 512MB PC2100 is better than the 128MB PC2700 that I have. I put in PNY DDR w/ Infineon chips.

What's the CAS latency of both the PC2700 & PC2100's?

I had a client with a Dell (might have been Gateway - I forget), anyway, the motherboard only liked CAS 3. Nothing else worked.