As me and many others do have problems with wifi cards and Windows 7 x64 (I can't get my card working on my Win7x64), what is the best option for wifi card for that OS?
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There is no problems using WIFI in Windows 7. Problems, if there are any, will lie with the hardware - or more likely with the drivers for the hardware. Either way the problem lies with the hardware manufacturer.
Virtually any USB adapter will work - but check first that the model is compatible with x64 OS's. There are PIC/PCIe/PCMCIA alternatives depending on your hardware, but USB is often the best, simply through simplicity and ease of installation.
Not knowing where you are makes it difficult to make a recommendation, but go to any major on-line store and they will have a dozen alternatives available to you. If you want to play safe, stick to a recognised brand: Netgear, Belkin, Linksys, Buffalo, D-Link etc... I particularly like Netgear stuff... historically it just always been reliable.
External USB Wi-Fi card could be an option or even PCMCIA.
You can find a lot on Tigerdirect or ebay. Site that sells IT stuff.