Want it 2 be main hard drive on desktop,

Hi,
There seems to be a misconception here. The virtual memory is a part on the Hard drive that your computer uses similar RAM. It only uses it once you have utilized most of your RAM so increasing it would not do much value at all. I suggest you upgrade the physical RAM sticks. (the memory physically attached to the motherboard of your computer). For your Seagate, it would be used just as a storage for data and not as memory. and since it is external, you could also not allocate some part of it to be used as virtual memory.
you could go to a computer store and ask them what memory (RAM stick) would be compatible for your computer if you want to do an upgrade. I'm sure you could also ask them to put it on for you to have the result or your desired amount of memory installed to your computer.
hope this helps.