I agree with 11team -- sell the computer and buy another one. What you have is just about the highest end Core Duo, which is why you have the 2GB limit, and I don't believe you can put a Core 2 Duo board in that case, or put a Core 2 Duo processor in a Core Duo motherboard. Even if you could, you'd be spending a lot of money and would end up with a hot-rodded system and no warranty, so it would be difficult to sell when you did eventually decide to upgrade.
It makes the most sense to sell the system and buy a brand new unibody 2.6 or 2.8GHZ laptop. It's the only way to get a serious speed bump.
FYI, I just checked, and the Apple refurb store has 2.66GHZ 17" Unibody Pros with 4GB RAM and full warranty for $1949! Pretty darn good deal.