You must have a graphics card with OpenGL drivers. On Windows you often need to download the drivers from the manufacturer rather than use the drivers provided within the OS (if you have an ATI or nVidia icon at the bottom right of your screen then this is done). The implementation of OpenGL by Matrox seems buggy and occasionally crashes. I would recommend nVidia or ATI cards.
The newer the graphics card the better. Specific improvements came with OpenGL 2.0 in 2005, supported on nVidia FX5200 and ATI 9600 cards (or any since then). Some features of PsychoPy are only available to cards that support OpenGL 2.0 (for PsychoPy 0.93.0 it is/was a requirement and may be so again in the future). You can buy an nVidia FX6200 for around £25 and it is worth it!
Faster and preferably multi-core CPUs. A faster CPU will allow you to draw more stimuli without dropping frames. A multicore CPU is less likely to drop frames when other things happen on your computer (like a backup starting or a virus scan).
Standalone audio cards (e.g. Sound Blaster Audigy) are a plus if you plan to use auditory stimuli. On-board (integrated) audio can have very substantial latencies.
If you're buying a PC it's worth trying to get one with parallel and serial ports if you're planning to interface with other hardware. It can be very handy to be able to drive hardware and check TTL signals from parallel ports. Attaching one via USB incurs a latency (USB is fast but delayed).
