First of all, I highly suggest that you look at the Pentium 4 641 (Cedar Mill) and Pentium D 920 (Presler) instead of the 640 (Prescott) and 820 (Smitfield) as the 641 and 920 are cooler-running 65nm parts.
The 920 would do much better at multitasking that the 640 will, but the 641 would do better in games as it is 400MHz faster. For multimedia playback, both are more than enough, and for encoding, look at benchmarks. The multithreaded encoders will go much faster with the 920 and the 641 would do a little better than the 920 in single-threaded encoders.
I would personally recommend the 920 over the 641 as I think it is the more future-proof chip. Both run in the same motherboards, and a 945, 955, or 975 chipset will run them. I'd recommend an Abit, ASUS, or Intel board for either chip. (I have an AMD system, so I can't recommend specific board models for you as I haven't looked at them much.) Put 2 1GB sticks of DDR2-667 in there and you would have a pretty fast computer.