what model Dell is that? if it just warns you but still runs with no fan on the CPU header you may as well do that and just hit enter each boot. the thing is, obviously the duct fan = the CPU fan and noone sane would run a reasonably modern CPU with no fan (unless it's a 3 pound copper tower or water or something), Dell doesn't want grandma's PC to die when her fan chokes on dust, and there you go. you *might* be able to get away with only splicing the RPM signal onto the Dell connector and using a regular molex for power, for your fanbus or a 7v mod or whatever, but i've heard some Dells won't run without the mb both getting an rpm signal and also actually providing the power to the fan.
short answer, splice it, no reason not to, especially if the stock fan is dead/dying - if it's not you might want to just leave it depending on brand. Those NMC/Datech 92mm fans are pretty decent, assuming your Dells don't have panaflos; most seem to use panaflo or datech CPU fans and Adda PSU fans, which are also very good. Also remember if older = P4 <2ghz or so, the fan's current speed control is probably via a thermistor on the fan, not the mb.
Those connectors sure do remind me of those old 3 pin internal audio cables though, but you'd still need to solder even if you could find some. The connector on the motherboard *could* also just be friction fitted (like most PS connectors), if you're careful you might be able to just remove it and try to fit a regular 3 pin connector on it.
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