Hi:
You should be able to.
On an older PC like that, I would suggest you get an Intel 535 SSD.
They have come down in price recently.
If you are going to install W7 from the HP recovery disks, you should get a SSD that is around the same size as the one you currently have.
If you are going to install W7 from a plain W7 installation disk, then it doesn't matter what size SSD you get.
Note: A SATA III SSD will only run at the drive controller's maximum rate of 3 GBPS on the dc7900.