Adding memory will not instantly increase the heat of your system by 40C. (assuming that your processor was running at a typical 40 to 55C temperature to begin with)
I agree that you should check to make sure that you didn't knock anything loose during the installation...like the CPU fan connector!
If the CPU fan is still spinning, then try removing the extra memory and take a look at your temperature. If it's down in the normal range, then you probably have some type of interference problem. I've seen this sort of thing before, though never caused by memory. In this case, first remove your old DIMM and put the new DIMM in slot0. Boot up and see if your temperature problems disappear. If they do, put the old DIM in slot1 and boot up. If your temperature is normal, you're set.
If the new DIMM causes temperature sensor problems, then send it back. If the COMBINATION of the new DIMM and the old DIMM cause the temperature problem, then determine which of your DIMMs is higher quality and order another stick of the same model from the same manufacturer....then return or sell the "bad" stick. Then recall how I'm always whining that you should only buy name brand CAS2 memory
If reversing the DIMMs solves your temperature sensor problem, but you still have lockups or strange explorer crashes, then either your motherboard isn't capable of effectively driving that much memory, or there is a problem with the combination of the DIMMs you are using. Some things you can try: 1) use only one stick of 512MB 2) use 2 "identical" sticks of 256MB, preferably high quality CAS2 3) move to registered DIMMs (motherboard support required) which aren't much more expensive these days.
Regardless, I have to be a dick here and ask you WHY you went to 512MB. I only have 164MB and even recording in Win2K I have yet to come close to consuming all of my physical memory...in fact it is rare to be over 128MB even when working with a lot of plugins. Now if your doing graphics or video work then chewing up memory is pretty simple...but for recording and everyday use, even gaming, 256MB is more than adequate....in other words, I know that memory prices crashed and I know it probably seemed cool to jump to 512MB for what, sixty bucks? But is it worth the headache if you're not actually gaining anything at the moment? Of course I'm not aware of your situation, so if you really are sucking that much memory, then please disregard this paragraph.
Oh, I guess I'm rambling here but one more thing.....if you REALLY DO have a temperature problem, and it's not just the sensor...then you'll need to take the above advice and check your heatsink seat and the fan. A typical CPU fan will spin at 4000 to 5000RPM's. And be aware that there is no way to check the heatsink seat without completely reseating it which requires patience, thermal compound, and something like nail polish remover to help you remove the existing grease or pad.
Slackmaster 2000