As fast as possible! Of course with budget restraints, you may not be able to buy the best machine. Using Passmark's CPU benchmarks, it is good to shoot for at least a 1,500 score. http://www.cpubenchmark.net/ Processors like the Core 2 Duo from Intel are a good choice. See the next question for a little more detail.
Future Pinball and Visual Pinball will both run under either XP or Vista, and presumably will work fine under Windows 7. As far as performance under XP / Vista / 32/64 bit etc I don't know. If anyone has any comments regarding this question, please let me know.
You should stick with a high end graphics card that will render greater than 30 frames per second, which would be your absolute minimum. Nvidia seems to be the recommended card brand, with at least 512mb and at least the 9800 series. My initial test steup, a GeForce 6200 running on a Pentium D 2.8ghz with 4gigs of ram (a PC purchased in 2005) runs about 25fps in Future Pinball set at medium graphic levels, this is unacceptable, so you should shoot for much better with your configuration. The Ultrapin, made by Global VR, used a Intel 945 Motherboard with a Pentium D 3.0ghz processor, 1 gigabyte of RAM and an Nvidia GeForce 7600 GS, running on XP. By their own admission, this was not strong enough to run Future Pinball with acceptable framerates. For more information on the history of Ultrapin, read here.
Some people just use screws with spacers, others use the tray from a case.
POWER SUPPLY
Make sure that you purchase a power supply that is sufficient to run all the graphics cards in your system. Your card may need more than 400 watts, so go with at least a 600 watt supply.