I seem to have a problem of denting the rims on my rigid 1FG. Is it worth it for me to put disk brakes on this light thing just to be able to have a smooth braking surface, or should I just invest in a stronger set of wheels?

If I were to go to disk on front and rear, what would be the lightest cable actuated that I could get?

BTW, I'm 6'2" and about 173 on an XL frame.

First, I'd invest in a set of high volume tires like a WTB Motoraptor 2.4 - it's not a total pig like some of the bigger tires. Also, watch out on measurements, some tires claim 2.3 and measure 2.1 - personally I say go for a 2.3 actual size minimum.

Weight: Disc brakes rock. For cable actuated, Avid seems to get the best consistent reviews hands down. If you are not tied to cable, I recommend a Hope mini, light weight and good stopping power. If you are really worried about weight, look carefully at the hubs, this is what will make or break you.