Short answer: no
Long answer: You shouldn't dedicate all of your RAM to minecraft, as it won't help, and will likely just hurt you more. You only have a certain amount of RAM (in this case 3GB), and basically, you use RAM to run your computer. You need a chunk to run the operating system, games, internet browsers, you name it. If all your RAM is allocated to your minecraft, it's not going to be available for use on anything else, and you really shouldn't need more than a gigabyte allocated to run tekkit. The reason it's freezing up (unless it specifically says "ran out of memory") is likely nothing to do with RAM. Also, the reason your iMac's options for RAM allocation can go up to 16GB is likely because the java on that is 64-bit, as opposed to 32-bit, though I could be wrong on that. As for preventing it from crashing, you could try to set all the graphics options to the lowest possible (if you havn't already). If it continues crashing, you can just time it, and manually restart the client before it crashes, so getting back into the game will take less time.