Cardio is great for your voice. I'm not sure about weight lifting, but i'd assume it would be okay(look at Jim Gilette, he's jacked, haha).

weightlifting is fine. running is the best exercise for your singing. i'm talking proper running here, ie. not just going out once every now and then and going through the motions. the key to progress in ANY skill development is seeing each session as a battle where you must outdo what you did last time. ie in weightlifting, beat your last lift by a rep or two, and once reaching your target rep range add more weight on the bar. for running, try and beat your last time by a few seconds or so. it doesn't sound like much but if you are truly working HARD then you will only be able to beat your last score by a little bit because your always pushing yourself to the point where you can't continue.

as for making running fun and motivatingly hard: here's what i used to do and will be starting it again soon:

i made a song that goes for 25 minutes (remixing metal gear solid music as it fits the theme), i mapped out a 5km path which is 3.10 miles for your americans. running at nighttime as theres no cars and no distractions. this is a death run because its ******* hard. basically you have to finish the 5km run before the 25 minute song finishes. if 25 mins is easy for you obviously you make a shorter song. there are sections in the song where the pace and tempo picks up and you must match the intensity of the music. basically your pace is a quite fast run and then some spritning now and again, NO REST. YOU WILL WANT TO QUIT BECAUSE YOU FEEL YOUR DYING BUT THIS IS WHAT HARD WORK IS ABOUT. Near the end of the song the intensity goes really high for the last 3 minutes. I remember one time i was about 1.5 km away from the finish and i was SCREWED felt like dying, and suddenly the last section of the song came in, i knew i only had a few minutes left, I HAD TO JUST SPRINT even though i felt like dying I HAD TO DO IT. it was the worst pain ever, but i beat the song JUST. every session was like this, but eveyr session i would beat my last score by a few seconds. this type of training develops you in so many ways not just physically but your overall character. over a few months your gains will be quite considerable. this is the same with weightlifting or any skill development. consistency and hard work and adequate recovery time. in the future someday i will be writing a book on true strength/health and skill development.

when you do weightlifting or running or anything in this type of mindset where each session is an actual battle you will actually get nervous before each "fight" you'll get a bit of adrenaline going through you before you head out and you'll want to give yourself about 30 mins before you head out some time to concentrate and almost meditate on the upcoming "battle", it sounds stupid but when you really do push yourself like this this is the truest truth of making progress and this is how the world's strongest and most powerful athletes make their progress, don't see it as "exercise" see it as a true battle to improve upon last time.

btw if you want a great singing warmup, learn to do headstands. that is, do a handstand against a wall then lower yourself and balance on your head, it is an incredible neck strength developer (my friend who has a thick neck is in the guinness book of records for the most headspins from doing this for years link is here http://www.youtube.com/watch?v=85mTrHB7abw) now i can't spin on my head but just standing on the head is great for warming up and if you want to go the extra distance and do timed holds until you can't do it no more its great for neck strength development. it will get all the blood going to the head and you will feel great for singing when used as warmup

When you lift weights, inhale during the easiest part of this exercise and exhale as you apply you strength to avoid sending the pressure up in your neck. Send the pressure down just likt the power push.