OptiPlex GX280 is a year 2004 model with the Intel 915G chipset.
Although there are USB 3.0 add-on cards, there is no PCI express 2.0 slots on a model that old. Without PCI express 2.0 the transfer rate would not be able to achieve a speed close to what USB 3.0 has. Data just can't move thru the PCIe lane that fast.
•A PCI Express 1.x lane can transfer up to 250MB/s
•A PCI Express 2.x lane can transfer up to 500MB/s