I was surprised at how easy this actually was.

I watched a YouTube video, and there was heatsink compound that needed replacing etc., but that was not the case with mine?!

A precision philips head screwdriver, and small alan key, and I was in, only 4 torx screws to undo, all quite easy (there is some locktite sealer on one or two screw threads). Simply replace the 168-pin DIMM.

I replaced the existing 128MB DIMM with a 256MB one. I now have (only) 768MB. I have sourced a 1GB DIMM, so will look into ordering that.