I've been using a 3Com product similar
to the 3Com® Wireless 11a/b/g PCI Adapter
to provide 802.11a connectivity for years without any issues:
Don't be put off by the website showing compatibility with XP/2000. If you check the Windows 7 compatibility list from Microsoft you'll see from [url=http://www.microsoft.com/windows/compatibility/windows-7/en-us/Details.aspx?type=Hardware&p=3Com%20802.11a%2fb%2fg%20PCI%20Wireless%20Network%20Adapter&v=3Com&uid=3CRDAG675B&pf=0&pi=1&s=3Com%u00ae%20Wireless%2011a%2fb%2fg%20PCI%20Adapter&os=32-bit]this page
[/url] that the "product installs automatically without extra software".
Although I only use it for 802.11a it also works just fine for 802.11b (I've never tested 802.11g). My 3Com hardware currently runs to two gigabit switches, an 11a/b/g Access Point, the aforementioned PCI card and an 802.11b USB adapter and they all play very nicely together and have been ultra-reliable (but I did have one failed "power brick" on an previous 100Mbps switch).