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It doesn't sound like an underpowered PSU to me. 400W should be enough if you don't have a power hungry video card, which from your description isn't the case. See e.g. Intel Core i5-2500, Core i5-2400 and Core i5-2300 CPU Review. Page 10 - X-bit labs.
That review measured the power consumption of an i5-2400 system under full load as < 100W. Of course you need more than that, but if you only have a CPU, one HD and a DVD drive, then probably not that much more.