It probably is. However, I'd check the rails with a multimeter. Software takes it's readings from BIOS and may compensate---or attempt it---for in accuracies coming from the BIOS readings. The BIOS readings aren't off the unit itself, they are not accurate. They read after it's passed through some components, leaving it less than the full amount. BIOS tells me my +12V runs at 11.8, but a multimeter registers it at 12.01-12.03 or so. This is my Enermax 535W. My OCZ Modstream 450 gets about the same crap.
Secondly, a tolerance of 5% in either direction is acceptable. If you're not going under 11.40V, it's within tolerance. But, meaure it with a multimeter.
While I do think the power supply should be replaced with what you have, definitely test it just to be sure. Are you getting BSODs or not? If you are and they're different ones, that's a good sign you have a RAM problem.
The Pentium D's use a lot of juice and the X1950 isn't exactly light on power usage either. Some 450W might pull it, but that depends on how much juice the rails have. Even if the unit only uses 450W, running the PSU at full throttle all the time is not a good thing. You wouldn't run the car engine in the red all the time and expect it to last 20 years would you? Same thing with power supplies. Running them at full capacity isn't going to have them last for a long time.