"Please tell me what is the correct nomenclature for the 2GB RAM stick I should buy for my Gateway LT2802U?"
PC3-8500 DDR3-1066 204pin SDRAM SODIMM is the correct specification of the memory.
You can find the memory from the following link (if you still need it) - http://www.memorystock.com/memory/GatewayLT2802uNotebook.html
Upgrading memory is the easiest way one can increase the performance but I think the processor is not good enough so if I were you I would go with a better laptop or perhaps a mobile workstation depending on the budget. But good luck anyway.