eating a "low carb diet" will only mean you have less energy for gym work and rugby, since carbohydrates are a slow release of energy and is the main thing our body looks for to supply us with energy. My advice would be to first go on a bulking up programme, since you are playing rugby i would only stick to a 3 day a week maximum workout plan. For bulking up and gaining weight rest for your body is absouloutly vital, if you feel you are still tired before you start another gym session then try going on a 2 day a week programme. While bulking up protein is essential as this is the main nutrient for growth and repair of musles, working out hard is usless if you dont plan to let your muscles repair and thus grow bigger. After you reach around 10 stone, or you feel like you want to remain at that weight, then switch to more exercises aimed as muscular endurance, especially around the abs. But the key to getting a "6 pack" is losing fat, so aerobic cardio work should be good eg. jogging, but to be honest a 6 pack is not going to help you in rugby just maybe help with the ladies, but even then its more impressive having big muscles rather than a six pack :) Hope this helped, check out the websites below for help if you wish to start weight lifting :P
Bascially a high protein, low carb diet and a sensible strength training routine. There are a million variations of this theme, but in the end that's what it boils down to.
Eat a lot gym alot_ work your abs
