Unless you have network cards and routers from the same manufacturer, the high speed will not work, as each manufacturer's high speed technology is non-standard.  The 52 Mbps speed of standard 802.11g routers is plenty fast for internet browsing and the higher speed is only needed if you frequently transfer large files between the computers. Note also that it is always a good idea to have at least one computer on the network that connects by a wired ethernet connection to the router.  This way you still have access to the router to configure it for wireless connectivety and troubleshoot any problems with the wireless.   Thus, you really don't need a wireless adapter in the computer that is located near the modem and the router.  Note that nearly all wireless routers have 4 ethernet ports to connect up to 4 computers by wired ethernet.

Steve 

Since you have not made it entirely clear which computers have which devices and exactly how you plan to connect it all up, I cannot give a definitive answer.  If the computer located nearest the broadband modem has an ethernet adatper and if the modem can connect by ethernet, then I would recommend getting a Linksys 802.11b/g wireless router and connecting the modem and the computer that is nearest to the modem by ethernet cables.  Connect one cable from the modem to the WAN port on the router and one cable from the ethernet port on the computer to one of the LAN ports on the router.  The computer located farther from the modem will need a PCI wireless network adatper and will connec tot he router through wireless.  If both the router and the wireless network card are from the same manufacturer (i.e. Linksys) and both support that high speed standard then the high speed mode of the router will work.  If you don't plan on frequently transferring large files between the computers, then there is no need for the high speed and a regular 802.11b/g router and wireless network adapter will work just fine and save you money.

Steve

Does the computer connect with a connector that looks like a larger version of something that looks like a telephone connector (an ethernet cable) or does it connect with a flat metal rectangular connector (a USB cable)?  If it connects with a USB cable, look closely at the modem to see if it has another connector on it that allows for an ethernet connection.  In order to connect the downstairs computer to the router, you have to be able to connect to it by ethernet.  On the computer upstairs you will need a wireless network adapter. 

Steve