Yikes, a generic PSU.
Get rid of that thing and pick up a Seasonic.
I am using a Seasonic Super Tornado 300 (that's right, just 300 watts) in my main HTPC rig, which has a P4 3.0gHz Northwood and the same Zalman 7000 you have, Abit IC7 (i875 chipset), 2X 250GB SATA HDs, 2X optical drives, and a Radeon 9800Pro plus Hauppauge PVR150. All in a horizontal desktop HTPC case. My case temps never rise above 31C when room temp is 25C.
I've never had any PSU-related instability problems.
Kill two birds with one stone. Get a Seasonic-- Better power and runs cooler.