[Sephiroth]

A decent wireless router is designed to do what you want. But the SH is a wretched piece of router junk. It has two postage stamp sized plates in exactly the wrong place facing the wrong way in a most beautifully crafted useless box. Tis thing will never do what you need for wireless (although it might for others who are in the same room as this thing).

Modem mode and your own wireless router is the only answer. But think about the router you buy and read up on its wireless legs. For an upstairs/downstairs thing, there are variables. A router with an external antenna pair/trio would to a better job than one with internal antennae.

Hope that helps.