When I go on Rollercoaster Tycoon 3, it comes up with an error box:?

Most graphics cards aren't that cheap these days, and the cheaper end cards may cost around $50, with more expensive ones ranging up to $600. I would recommend an eVGA e-GeForce 6200 TurboCache 32MB if you are looking for a slightly cheaper card with moderately good performance. Try asking your dad to test his graphics card in your computer, and offer to swap if both computers work for the swapped graphics cards. Your dad should not need an advanced card for most computer business activities, so try doing this.