I had an almost identical experience last weekend. I upped my RAM from 1Gb (512Mb x 2)to 2Gb (512Mb x 4) about a month ago using identical Corsair matched pair memory. All seemed well until I switched on PC last Sunday and got as far as a message saying that my PC failed to shut down correctly last time (not true) and gave a selection of possible boot options such as Normal Windows, Safe Mode, last known good config etc. Whichever one I selected always lead me back to the same error message. Eventually I removed the new memory modules and all went back to normal. I propose to test the memory this weekend (will look at previous threads to see what is recommended when time permits) to narrow down where the problem lies.
I realise that this is not really helping your own problem, but at least you know that I feel your pain! Sorry.