Thought I'd post an update. I had hoped to get more feedback here, but I guess what I was trying to do isn't as common as I thought.
So, I have my set up running now, and I'm actually typing this from my living room in front of my Samsung 40" LED tv on a wireless keyboard. I'll list what I have set up.
First, I sent inquires to 4 different companies asking for recommendations, and ONLY Techorium responded, and quite promptly too. Naturally, they are the ones that got my business. Was very disappointed with the lack of response from the other three.
Here's what I have set up:
NTI DVI Extender with Stereo Audio
Icron USB 2.0 Ranger 2204 - 4 port USB 2.0 Cat5 Extender
Each device I have connected to its own dedicated Cat6 drop, running approx 50ft to the upstairs PC.
My resolution is 1680x1050 (my video card doesn't support 1900x1200, only 1900x1080, which distorts with a too-wide display). I currently have both keyboard and mouse running through the USB hub. Audio is coming through a separate connection on the DVI extender.
I haven't tried connecting my audio card through the USB extender yet, will attempt that soon. It was for the audio card purposes that I got a USB 2.0 version instead of a USB 1.1 extender. Big jump in price let me tell you, but I needed the extra bandwidth.
So what are the results after running this set up for two weeks now? Beautiful. I often forget that my PC is actually upstairs. There is a miniscule delay in video/usb performance, but I only notice it if I pay attention. I've even tried playing Torchlight with this setup. Again there is the odd artifact that shows up in rendering, but for the most part its smooth.
One very minor drawback, there is no monitor detection with this set up, presumably because the DDC info isn't sent through a single-cat5 type device. I had to manually adjust the resolution, but only the one time at first connection.
Total cost for this before taxes: $823.10 CAD. Pricey, but I'll be getting a lot of use out of this for a long time, so worth it to me at least.
Thanks to those who responded :-)