RE: 5*5 weight
The first important thing is you want to be able to keep proper form on all of the reps with the weight you're working with. If not, the weight is too heavy.
Secondly, if I remember correctly, you want to have at least 8 reps altogether on your last 2 sets(i.e. 2 set of 4 or 1 set of 5 + 1 set of 3), otherwise the weight is too heavy.
Thirdly, its usually better to start out with a weight that you know you can do 5x5 and work your way up from there. This way you won't end up wasting a training session with incomplete sets on the exercises due to having to drop the weight because it was too heavy. Afterall, its always better to go up instead of down. Also, less risk of injury.