You may be able to add a second hard drive, if there is space to allow it and if there is a second connector on the cable; if no second connector, you can add an extra channel via a pci slot.
Did crucial not tell you the max memory possible? Probably 1gb in each slot would be my guess.
The internal USB slot would be to allow the addition of another device such as the card reader you suggest.
All in all, it's best that you open up the machine and take a look at what space is available.