I would add that using a Mac with only 10-15% of HD space on the boot drive is not a good practice. I understand that you don't want to move your iTunes library to your other drive.
You could simply purchase a larger new HD and use Carbon Copy Cloner (free) to clone your 80GB to the new drive. I don't know where you live but HD prices are really low, around $60 for a 500GB.
Back to your problem, wolfnowl is right, when you import your photos, Lightroom gives you the choice of copying files from your memory card to any place you choose on your HD.
By the way, if your Lightroom library is located on your boot drive, you might also have a space problem as previews generated by Lightroom will eat the space very quickly.