I'm pretty much a novice when it comes to upgrading computers (have replaced power supplies and graphics cards, but that's it). The computer above comes with 2Gb (2x1GB PC2-5300 MB/sec) occupying two of its four 240-pin DDR2 DIMM sockets. According to the hp motherboard page it supports PC2 3200 (400 MHz), PC 4200 (533 MHz) and PC2 5300 (667 MHz) DDR2 DIMMs, non-ECC memory only, unbuffered. My research has revealed that UDIMMS = unbuffered DIMMS. The same page states that the maximum HP/Compaq approved memory is 4 GB and that 32 bit PCs cannot address a full 4.0 GB of memory.
I am planning to backup all my files and reinstall vista as 64bit which I think I have worked out how to do (although the bios issues referred to in this thread on the hp support forum thread suggest this might not work in which case only about 3.5Gb of memory will be addressable anyway, still better than now. I'm also planning to put in the fastest supported CPU and a new cpu cooler, plus a much better PCI express X16 graphics card (I have 1Gb and 2Gb cards available and will research which to use depending on total memory addressable). I have bought 4Gb (2x2GB PC2-5300 MB/sec DDR2 DIMMs, non-ECC (edit - unbuffered) memory) with the intention of expanding the RAM to 6Gb (with my existing memory).
The thread above mentions the possibility that adding more than the recommended maximum of 4Gb of memory, and using 2Gb sticks in a board designed for 1Gb sticks might damage the motherboard (although the questioner in that case had done both successfully). My question (to which I have not yet been able to find an answer) is, how might this damage the motherboard? Finding out the answer to this will help me assess the risks involved before trying it out.
My research reveals only rather vague answers of the type "no, it won't explode! :)" (answer to Superuser Q) or "possibly damage the ram and motherboard. DO NOT DO IT..." (answer to Tom's Hardware Q) to "most system these days protect from such mistakes" (answer to Spiceworks Community Q).
I hope this is a suitable question for this site and that I have met the research requirements successfully.