I suggest the pull up to the nRESET signal be tied to +3V3 (which +3V3?).How about a MAX6454 uP supervisor device connected to the +5v rail from the PC PSU?
Maxim/Dallas make very good watchdogs. There are several other vendors, so we should try and find an easily obtainable, multiple-sourced part if we can. I have had good luck with the Microchip MCP120 series of late.
For example, the Microchip MCP120T/460I (4.6V) and /450I (4.5V) are each 48 cents from Digikey, and a drop-in equivalent part is available from STM, the STM1001MWX6F, for 77 cents. I suspect they are similarly available Down Undah :-)
One of the problems is that most open-drain output devices have an internal pull up and if we are monitoring the 5V supply, we want the pull up to go to the 3V3 rail. Our choices increase with a monitor on the 3V3 local supply because we can then tolerate an internal pull up.
Lyle