I finally found a solution that works for me. Unfortunately, it doesn't involve Linux Mint.
After trying OSSv4, ATI propietary drivers, and all of the other stuff I've mentioned in this thread, I stumbled across a blog of a guy trying to get his family to use Linux (like me) and they were rejecting it because of a flash game on Facebook called Yoville. Yoville is made by the same company that makes the game my daughter is complaining about.
The blogger found increased performance using WINE, but eventually found the best performance using Seamonkey. I tried both solutions with Mint 7 XFCE and neither of them worked. In fact, just a simple Youtube video was chappy and laggy, with frames missing.
So I installed Xubuntu 9.04 and then Seamonkey. I instantly saw an improvement in performance, as did my daughter. But she wanted me to keep looking. It was faster than on XP, but it still wasn't fast enough. So last night I installed Chromium and Midori. I eliminated Chromium instantly. It performed exactly like Firefox and such on Mint, even on Youtube.
And then I tried Midori. For anybody not sure what Midori is, it's the "stock" XFCE browser, similar to Konqueror for KDE or Epiphany for Gnome. I'd read alot of people saying that Konqueror and/or Epiphany were their fastest browsers, so I thought I'd give it a shot.
My daughter loves it. She gave me the thumbs up. Even Pet Society, which is the slowest game on Facebook, renders smoothly and quickly with no stuttering, even on high detail.
Unfortunately, the performance is only that good on Xubuntu. On Linux Mint, it's still laggy.
