Your computer is capable of handling 64 bit, it is not have a 64 bit processor. You can use 32 bit on any computer, although some memory may be wasted.
When you buy an OS, you are mainly paying for the key, not the DVD. You may want to call MS and see if you can get a DVD for cost plus a little more. You may, also want to contact your vendor to see if he can help.
You were probably sent 32 bit because you had 32 bit alreaday installed. If it was an upgrade DVD, you cannot upgrade from 32 to 64, you must do a clean install. Note, however a clean install can be done with an upgrade DVD.
Wish you luck