I think it's always worth optimizing for any keywords you can EASILY get top page rankings for. The reason being that even one extra sale will often justify the *tiny* little bit of effort it took to rank.
Once you've taken care of the easy stuff, go after the big terms. Did you also explain to them that they can have *instant* traffic via PPC ads? This is also a good option.
A lot of people like the idea of spending cash on instant PPC traffic while they wait for the organic traffic. Also the extra traffic will give you an opportunity to optimize the SALES COPY! A lot of traffic is useless if it can't convert.
So to recap:
- optimize for the easy stuff
- start optimizing for the tough stuff
- PPC
- Convertion testing/optimising
- Enjoy the free traffic!
Pete.