SIS real256E graphics - This is onboard, and you'll get awful gaming performance even on the lowest detail in game.
She should have enough ram but the onboard system does share this with the system. A game like Rollercoster Tycoon 3 isn't over the top on graphics so you should only need to spend £90 - £120 on a graphics card.
Make sure you check the PC has an AGP slot on the motherboard. I'd consider another 128mb ram too for gaming depending on how clean and tidy the pc is kept.