I get those spikes too... I wondered about it too, then I defragged and tried it again and there were less of them, so I think it has to do with fragmentation somewhat.
My board is a KT400, and your's is an nForce2, and we're getting high CPU utilization... so that tells me it's not the chipset or drivers. Unless both chipsets have faulty IDE drivers. But I doubt it... I wonder if it has something to do with the drive itself? Where did you get your's?
I just saw benchmarks on THG showing the WD1200JB using less than 8% of the CPU during their tests.
It's strange... that's pretty much the only way I notice a difference... when I search for or copy files and watch the Performance meter in the Task Manager CPU utilization doesn't get that high. But when I run HDTach and watch it, it goes to 100% and stays there till the test is finished. Other than that, my benchmark scores are fairly good considering the componants I'm using, so I don't know... maybe it doesn't have that big of an effect?