Fog lights serve a completely different purpose that high power headlights.
They are meant to be mounted low and have a very sharp cutoff. This allows them to shine under the bottom of heavy fog (which almost always holds a little bit above the ground due to the warmth of the pavement compared to the air temp). Doing this avoids having the light just glare and flare at the front of the vehicle the way headlights do in heavy fog.
Simply making your headlights brighter just gives more glare.
Fog lights also help (although not as much) in heavy snow. Mainly because the low positioning and sharp cutoff help to reduce dumping all the light into the snow at eye level and provide less obscurence due to this.
Remember... fog lights are not driving lights and driving lights are not fog lights.
IF the OP is actually talking about driving lights... a $150 set of driving lights is far more effective at lighting up the road ahead than any bulb upgrade for your OEM headlights.
I look straight at Japan from my couch, ie: coastal north pacific. We get nothing but dark rainy and foggy drives to and from my place on deer infested country roads. I would be scared (not really) to drive it without my PIAAs. They are the yellow lens, like the F520 series fogs. They are freaking awesome. But, cost more than $150. For that price I would look at Hella 500s. They are a very thin 6" round fog. They are not as good as the PIAAs, but probably the best quality you will find for that kind of money.
I would highly recommend PIAA, very tough casing, wire harnes are top quality, excellent switch. I have own a couple pairs and even had to warranty a wire harness and it was no problem. Their Ion plasma lights are very good, clear when viewed by oncoming traffic but yellow when hits the ground.
I bought a cheapo set of Hella 500's, hacked them up, and installed a $30, 55 watt HID kit in them. I've got less than $90 in them , and they put out wayyyyy more light than any non- HID light. just my $.02