I'm sorry to hear of your problems.
Could it be incompatibility with your Router?
I'm running a DLINK wireless router connected to my 8MBPS Cable Connection (Just moved and they don't have FIOS... man... got REALLY used to that 15MBPS Connection!!!)... and my download speeds are almost 1 Meg per second... through wireless...
The Router is 2 rooms and about 35 feet away, fwiw.
The reason I question the compatibility issue... is... A) It would happen on both Boot Camp and OS X if there was a wireless card / router combo that didn't like each other for some reason... and... B) I have a weird wireless issue myself... and I think it's something weird with the DLINK router I'm using... IF I SET THE WIRELESS BROADCAST TO NO ENCRYPTION... it works fine both in BootCamp and in OSX...
However, if I set up WEP or WAP... It connects great in OSX... but XP through bootcamp NEVER ASKS ME FOR A USERNAME AND PASSWORD when trying to connect to the wireless network, and therefore can't connect.
DLINKS tech support was about as helpful as a sack full of lead weights to a drowning person...
So... my point being...
There might be some weird interaction going on between your Mac Pro's wireless adapter and whichever router you are using.
You might try a different brand (audition temporarily) from a store with a 100% Return Policy... lol.
That said... also make sure your Router has the latest firmware patch applied.
Cheers!