Some funny posts here.
Seriously, though, it's strange to have the pain in your upper arm. Perhaps if you get a chair with an armrest it would relieve the stress. Also, it helps to have the mouse and pad on a lower surface near your lap than up high on top of a desk (in my opinion). Also, it helps to have a good mouse which you can raise the sensitivity of so that you don't have to move your arm so much. Though, I guess it also depends on your grip style... of the mouse, you dirty-minded people!
I use a fingertip style and my arm never moves; I just flick the mouse around lightly with my fingers to zip the cursor all over my screen while my elbow is resting on the armrest (great for RTS, not so much FPS -- but I hate FPS games anyway, hah).
This is the desk I use, with a very large keyboard/mouse platform to pull out near your lap:
http://media-cache-ec0.pinimg.com/736x/61/4b/f8/614bf8687a7b61ace279c1dbf4910f29.jpg
It's awesome and for many years, it's been the only type of desk I'll ever use.
This is the older model I had back when CRT's where used:
http://www.z-lineracing.com/upload/products/65_1_large.jpg
Back when I was going to upgrade to a wide flat screen, I was freaking out because it didn't have enough room on top and I didn't want to remove the top platforms since it would look hideous. I really didn't want another type of desk. Thankfully, I found the newer model above, which not only looked nicer, but had all kinds of little improvements throughout it.
I use to suffer from RSI a long time ago, but it was in my pointer finger on my mouse hand (from all the gaming plus working in AutoCAD the rest of the day -- lots of clicks). I obsessed over fixing it and did some research and made adjustments to my posture and equipment. It also turned out I was using the wrong type of mouse for my grip style (too big) and the sensitivity I was used to was too low, causing me to extend my fingers out too far and too much, since I anchor my wrist to the pad.
Also, another thing I did, initially, was switch mouse hands for a period of time (I always get ambidextrous mice, just in case I ever need to rest my hand again). It was at least doable with AutoCAD, so I could rest it during the day, but impossible with any real time games that require reflexes. But, with a turn-based game like Civ, it shouldn't be a problem to switch hands (if you're able to) to rest your arm.
