Hi there,
I have been battling with this myself. I just put my MacPro1,1 in the living room for HTPC duty and the built-in Bluetooth was good for about 6 feet with an original Apple Wireless Keyboard (it wouldn't even pair with the Apple Wireless Mouse).
I went to MicroCenter today and picked up a Class 1 Zoom Bluetooth USB transmitter for $15. It has a 100 meter range, according to the package. So what the heck, I bought it.
Here's the cool thing: as soon as I plugged it in, the internal Bluetooth radio turned itself off. I re-did the pairing on the keyboard, and I can comfortably sit across the room now and type with the original Wireless keyboard - solid as a rock. It does disable the option to wake the Mac with a Bluetooth device, but I don't care too much about that. I'm still using a Logitech MX900 Bluetooth mouse with it (haven't tried the Apple Mouse again yet) and it works fine.
Just FYI: I'm using Plex 0.8.3 and this program is the shizzle. Very nice. Hulu and Shoutcast work well, but not everything is rock solid yet.
Find a Zoom 100M radio and your problems should be over.
JP