Because you say you want to keep it for several years, I would buy the Core 2 Duo and 2GB RAM. If you thought you would only have it for a year or two, take the $300 now and put it in savings until you're ready to buy another one.
You'll enjoy having the extra speed from the C2D and the performance from the 2GB RAM. You'll also like having the option of 802.11n as that becomes more available over the next few years (or immediately at home with the Airport Extreme).