Sandista,
Yeah it is possible to get something out of a 15 pound dumbell but after you can do like 12 reps without a problem then it is pretty much pointless as far as muscle growth goes. You need to put a new stress on your muscles to switch it up. As far as joining a gym just invest like $200 for a bench with like 300 pounds of weights. That is what I recommend for a beginner just to get oriented for the first year or so. Plus you don't have to worry about getting to the gym or finding excuses about not going.
Mole,
The amount of weight you can lift doesn't matter much in my opinion. At least for me its all about how I look and how I feel about myself. At 150 and doing 20 lbs for 8 reps aint bad. That means you could probably get up 30's 5 times. Just continue to challenge yourself with heavier weights and more Volume.
Peace and good luck to both of you