Jim ,
You are gonna have a ton of fun.
Be careful on what type of tablet you get (some won't let you use anything but a special pen.) Check out cocoontech.com or similar for opinions.
What's best for me may not be best for you; but check out my setup later on in the My Home Theatre Section of the Forum.
Use the forum Search tool for "XReceive" and read all the threads (except this one)
Do a search on "Mannyman" for previous posts. Notice how I push the forum Search function?
You should not have issues making buttons in XLobby to tell PC1 to control your A/V equipment. It sounds like we have similar goals as far as on the intended use of the tablets. I also had buying a TouchScreen Monitor in mind before getting the 3500s, because of connection difficulties you mentioned,, and for all the advantages a seperate PC brings to the equation.
Depending on where your PC2 is placed and what kind you get you might end up doing a lot more then just a/v and lighting control (ie. email, im, web lookups inspired by something on the big screen, are easily done on the tablet without having to tax PC1.)
Even so for the most kick-ass setup check out Baddabing's HomeTheatre, he uses a connected TS instead of a tablet.
My implementation:
TouchScreen1 in the livingroom is my main remote control, it makes my PC1 (old p4 2ghz VAIO) in my den send ir to the 60something inch TV, cablebox, DVR, Denon amp, and sometime this month r/c curtains. TouchScreen2 does the same in the master bedroom.
When I enter the front door, I am greeted by gmail popup notifications on TouchScreen3 and check the video log taken by the eBay secdvr card (installed on PC1),
Once I caught my wife with shopping bags the day after she told me I should stop buying "electronical" devices to save money. That alone made all of it worth it.