I don't think the laptop versions of the Core i* have 3 memory channels (neither do the Socket-1155/1156 desktop versions), so 3x4GB of RAM would be a strange/suboptimal configuration. There probably isn't enough physical space in a 13" laptop for more than 2 modules anyway (unless you sacrifice battery or optical drive space). Even soldering them to the motherboard wouldn't save much space - I think all current 4GB modules use 16 DRAM chips.
The move from Core2Duo to Core i* alone should bag you a substantial improvement in processing power, at least; the lack of a quad-core model is presumably a battery life and cooling system trade-off.
The move from Core2Duo to Core i* alone should bag you a substantial improvement in processing power, at least; the lack of a quad-core model is presumably a battery life and cooling system trade-off.