Depends entirely how you're going to use your PC. As Semi says, if you are only going to have that one 400GB drive, and that's all you're going to have for the forseeable future, partitioning is best, as at least then if your windows installation fouls up you can blat just the C: drive, leaving the data on your D: alone. If, however, you can stretch to getting a seperate 100GB or so HD, having physically seperate discs for OS and data does reduce the chance of you losing everything at one fell swoop.
Personally, I've not used partitions in nearly a decade.