Just make sure it's a Northwood for s478. I like the 3.4ghz, fastest proc for it's time. Try and avoid ones with HT.

My goals are to have a cool as possible

Both Northwood and Prescott have a TDP of more than 80 watts, so you won't have much luck with any of them, because that's still a lot. The Prescott you have installed currectly should have a TDP of 115 watts, however. I'd opt for the "SL6WK Northwood HT \ 3.0GHz \ 800MHz \ 512Kb". That will avoid about 30 watts of heat and speed won't change much, since Northwood is slightly faster than Prescott at the same clock rate.

there's options like turning off HT or undervolting, if the BIOS offers that. i'm thinking heat should not be a big issue if it's just for quick benchmarking though, but otherwise only a core 2 AGP board could fit the bill.

I'd probably go SL6WJ, not because i care about the HT, I would care more about the 800FSB.
The current CPU is probably the fastest in practice, but may run hotter. The one I picked is coolest at that FSB I believe. Most of them should be within a few percent of each other, except maybe the SL6LA. I'm assuming at this speed none of the choices would be a huge bottleneck.
try a card or two with different cpus if you want and see if there's more than a 5% difference. I wouldn't consider less than that to be significant enough to worry about it.