I found antennas that improve range, but I don't see how they can be connected as my router seems to have a fixed antenna that cannot be removed. What products work with these better antennas?
Given the area you're trying to cover, I'd be tempted to solve the problem by installing two more inexpensive draft N wireless Access Points (APs). If it's feasible, use Ethernet to connect the WAN port of each new wireless AP to a LAN port on your existing wireless router. Two APs plus your router might create a large enough coverage area to reach your gaming adapters – although this depends on their actual locations and throughput requirements.
Anecdotal reports on the WRT600n suggest that clients perform reasonably well at distances up to 100 feet. You might try placing your router in the center of the building, with each AP 200 feet away, creating a combined coverage area (with a single SSID) that's roughly 600 feet long. If your users require very high throughput, this coverage area might not sustain fast enough data rates. However, this solution might just do the trick for most applications, with minimal additional investment.
Alternatively, you could look at other products that support high-gain antennas. A list of (older) Linksys APs and routers that can be outfitted with high-gain antenna kits can be found at the Linksys.com wireless home website.
However, the area that you're trying to cover will probably still need more than one AP. Adding two more draft N APs just seems simpler, and will let you benefit from other features in the products you've purchased. Note that you want to add APs, not routers, so that functions like DHCP address assignment and Internet access are performed in one place, no matter which AP any given adapter associates with.
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