220: single 64-bit memory controller, onboard video
415: dual 64-bit memory controllers, no onboard video
420: dual 64-bit memory controllers, onboard video
The D suffix means it has the deluxe version of nVidia's southbridge which has Dolby encoding/decoding. If you're not going to use the onboard video, then the benefits of the dual memory controllers are not too pronounced. If you do use the onboard video, then using two identical memory modules in the correct slots will give it more bandwidth and enhance performance, but it will still pale in comparison to most add-in video cards, being around a GeForce2 MX200 in performance even with two memory modules.
If you want a basic office system, an Asus A7N266-VM is a 220D board that I've had good results with. If you want to buy a new board for gaming or high-performance usage, nForce2 might be the way to go, since it will support the next wave of AMD processors.