If $$$ was no object I would upgrade the headlights (with harness) AND still get the aux.(driving) lights.
But if it is a concern, like with most of us, then I would pop in high-powered halogens in the existing housings (with upgraded harness) and a pair of decent driving lights and see if you're satisfied with that set up in various driving conditions.
If you still feel the need to get the fogs after that then you probably should get a pair, possibly mounted below/under the bumper to get the max. benefit from them, possibly amber/yellow in color (for rain/snow/fog driving).
I've got a pair of the Light Force 240's on my bumper now and I am planning on

rollyeyes) mounting a pair of projector fog lights in the bumper like what Steve ("airlaird") did to his ARB. I'll probably end up using them often, even when the weather/condition does not warrant it, just to allow me to see better by supplimenting (sp) the lowbeams.
If you don't really need the fogs, but just want to boost up your low-beam driving condition, another alternative (to spending the $$$ on (or replacing) the existing headlights, is to get a pair of assist lights, such as Sylvania's HID Xenarc's.
It's really low-profile, with wireless remote, and true HID (with ballasts).
They work fairly well and does not take up too much real estate on/in/under your bumper either.
One last thing I might mention is that I have seen a pair of aux. lights that function both as driving and fog lights (with H4 bulbs). I believe Warn offers a pair that meets such criteria which meets the European "E-code" standards that I might consider.
Although they might not offer as good of performance as the dedicated driving or fog units if the $$$ and the space (or lack of it) on your ARB was of concern then it might offer an attractive alternative with something like those.
ANYWAY, sorry for going on and on!
I would probably start with the cheapest/easiest and if that doesn't work, then spend more $$$.
Good luck!
Mot
