Merry X-mas!
1. A64s use DDR1-modules (184 pins), if that's what you have, they will work.
2. Yes, however if you plan to OC, it's best to have four similar modules, as you'll be limited by the slowest pair. Two GB of RAM might also be stressful to the memory controller, so it will probably cripple memory performance. To get high mem speed with one stick is fairly easy - there's a significant difference from one to two sticks. And probably even harder to OC using four...