I like the low-slung Vectras mostly for their cases (I have two). These come from a time when HP provided a power supply and case and left the motherboard to third-party manufacturers. In your case, the motherboard is an FIC VB-609. (If you want to upgrade to a PIII, let me know--I've got a couple of the Vectra VL motherboards a-mouldering away in my hellbox.
One gotcha is that HP contracted the boards out for special treatment of the power supply fan control. So if you replace the motherboard with a generic one, the PSU fan will run full-tilt making for a noisy system. If you try to use a generic power supply with the motherboard, some HP BIOSes will complain about a lack of power supply fan. There are ways around both of these issues.
For my own cases, I've got one hosting an overclocked Slot A board and the other, a Socket 478 P4. The latter needs the CPU 12V, so the PSU was replaced with a Seasonic model that fit the rather tight space.
