I upgraded RAM to 1GB in two 512 CL 2 DIMMS, single channel PC2100.

The removed DIMM was two 256 DIMM no name brand CL2.5

The clock speed difference is a decrease of 20 percent and a corresponding increase in performance.

I got the full 20 percent for the small cost premium for Kingston on the first DIMM.

Cptr boots faster, boots MS Office apps faster, internet pages via the internet temp files faster. Worth the $88 cost.  Overall very impressive performance increase.

The first 512 DIMM was the most sim cost effective.  The second, marginal improvements in CFS3 performance.  Worth it for a mid life upgrade.

FYI: Installation was difficult.  On second DIMM I finally shaved a little DIMM board off the notch area on the fat side.

You are right about shouldn't have the problem. 
It took about 45 min to get the first one in. Rocking it on it's long edge.  Had thoughts of taking it to a shop.
The problem was definitely in the notch. Might be the board peg being a little too fat but that can't be filed or that the DIMM notch is a little narrow.