Just tell him that your current computer is starting to become obsolete for the tasks you need it to do. Tell him that with a Mac (if he isn't tech savvy this works like a charm) you can run both Mac OS X and Windows "PC" things which is exactly what you need for school at the moment. Something along those lines.

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Lenovo D20 8 Core Thinkstation:
[Intel 5520 Motherboard | Two Intel Xeon E5504 @ 2GHz | 4GB DDR3 RAM | Mac OS X Snow Leopard 10.6.3 (running in full LP64!) | nVidia Quadro FX 580 Dual HDMI + DVI 512 MB| DVD +RW DL]

iBook G4:
[14" Mid 2005 | 1 GB RAM | 80 GB HDD | SuperDrive | Mac OS X Leopard 10.5.5]

∞