I would stay away from USB dongles and use a PCI-e or PCI with a dipole antenna with a db greater than 8
you could also add repeater for the downstairs. A repeater can cost around $50 or if you have another wireless router use it as a repeater
This is what a member of the family uses and it worked very well. But we have a repeater so no matter where in the house it is always 100%
remember, you will always get the most performance from wired. with wireless you will have lag spikes and reboots. some routers more often than others. The router we have had the best results with are buffalo airstation.