I would back up this computer and bring it to the computer lab, and ask for CPU replacement. It may also require motherboard replacement. Starting from some point, it is cheaper to buy the new system, rather than upgrade the old one.

Buy a new one

First off, you need a 2.0 GHz or faster CPU chip that is compatible with your motherboard. What socket does it use? Do you know? It's probably a Socket 775, but I can't be sure without knowing what your system is. Depending on how cramped your case is, you may need to do some serious disassembly of your PC. - Unplug the power cord then hit the power button to discharge certain components. - Unplug all other cords going to the PC - Open the case and remove any add-in cards like your graphics card or any other card that takes up a slot nearby. While not *strictly* necessary, it helps unless you have chopsticks for fingers. Depending on how much space you have to work in, you *may* need to remove the motherboard entirely. Many cases allow this by removing the right-side panel, undoing a screw, and sliding the motherboard tray towards the front of the case a bit. The motherboard is attached to the tray, but you can the tray attached to the motherboard. If your case doesn't allow that... it *can* be done, but it requires far more dexterity - Remove the CPU fan and heatsink. There are many kinds so I can't give specific instructions. - Remove the old CPU chip and replace it with the new one. BE VERY VERY VERY CAREFUL WITH THIS STEP!!!! - Apply a *small* dab of thermal compound (that white goop) - Re-mount the CPU cooler and fan. - Put your PC back together. Sounds daunting, but I've done it in under five minutes on cases that left me with plenty of room to work in. Now for a little secret; the CPU specs on those games (and most other software) are usually rubbish. The amount of RAM available and your video card have far more to do with gaming performance than your CPU. I run some of those "2.0 GHz-plus required" things in a 1.6 GHz just fine.