Now that it is getting dark earlier it's time to add extra driving lights. How many amps can be drawn from the alternator without causing problems? I plan on using a dedicated high current circuit and relay.

I have 35w on mine, they are powered direct from the battery, and the switch is powered from the accessories fuse via a splitter. I used to have 50w lights on there, but you just couldn't run them for more than a couple of days (and I use mine on all the time to increase visibility to others during the day) without having a flat battery.
The 35w ones run good. I try to remember to turn them off at traffic lights if I"m in a line that will take a couple of cycles to get through, or in very slow traffic. If I forget, usually I get a warning to leave them off to charge the battery, as it will start and the speedo screen will go blank when the battery is getting low. I have not been stuck with a flat battery since I've had them. I have to say though, the 50w ones were really really bright and lit up the road a lot better than what I have at the moment. If you have the money, I'd go for some flash PIAA ones that have the output of a higher wattage light, but still only draw low watts.

Right now I'm running proper H3 Halogen driving lights from pepboys, my previous version was somewhat like the Kuryakyn silver bullets, with the MR16 halogen bulbs (they ran 50W, then I brought some replacement bulbs from home depot for 35W, but the casing eventually fell apart as they were cheap knockoffs).