If I conect an external hard drive to my computer, can I designate a portion of it for upper level RAM usage?

The other replies pretty much said what I would have said.

Best use for an external hard drive is files storage. You could also use it for some transportable applications that you might want to use on more than one computer, but I definitely wouldn't use it for VRAM.

If you are wanting to use it on a desktop computer your best course would be to add an additional internal hard drive, which could be used for Virtual RAM. Not sure what you would do if it is a laptop computer you are using. If you are using a laptop and want to add or replace a drive you should talk to a laptop technician.

If you are thinking of Virtual Memory on an external drive, that would be the SLOWEST memory in your system. USB interface is slower than either the SATA or PATA interfaces used by internal hard drives. It will just be a slow exercise in futility.

Unlikely .. external drives wud be a USB, Firewire or eSata drive ... which wud be considered temporary as they are hot pluggable ... so it aint worth trying ...