The best way to warm-up for lifting is to lift lighter weights for a couple of reps. I.e. for your case, do 4-5 pushups, rest, bench 50 Kg, then bench 75 Kg, then go for your 'working' set.
You don't really need to do cardio work or stretching to 'warm-up'. If you're cold, yes, do some jogging or jumping-jacks or whatever to actually raise your bodies core-temperature. If you're pecs are really tight, you should probably stretch them, but it doesn't need to be pre-workout. Post-workout or whenever is generally better.
Check out the "Magnificent Mobility" DVD on this site if you want a really good way to stay flexible.