I'm into overclocking old Dells. I've got a couple workstations but I haven't run them yet.
T3400 QX9650, and t3500 W3690. I want to do a T7400 also. I'm gathering parts for it now. 2x X5470s. i couldn't find any support for the PLL chip on that one.
I really like what you're doing. I assume that OC is on stock voltage? My preferred technique is unlocked CPUs with Throttlestop software to control multiplier and Voltage. Although the T7400 will be BSEL and VID pinmods. The2006 Dell Dimension E520 in my sig. shows what this method can do. I sent a link from here to a guy at Tomshardware who is trying to get a W3690 to work in a T7500.
Keep up the good work.