I think 4 CPU is better since you have more processing power especially if your CPU are Quad core or higher CPUs and 3 GHZ and higher like the Intel Xeons CPU.
There is also more CPU Cache memory when you have more then one CPU. The fastest memory on your computer is the Cache memory on your CPU.
But, it will be a lot more expensive buying a computer with more then one CPU.
It also depends on your Operating System. I'm mostly using Ubuntu 10.04 LTS with the LXDE http://lxde.org/ lightweight desktop/theme Environment and disabling unused background programs which I never use like Bluetooth. I manage to tweak Ubuntu to use less then 100MBs of RAM when idle, and 1% of CPU power.
If you pick low ram usage programs like VLC media player, OpenOffice, Opera, Chrome, Foobar2000, a lightweght antivirus like Vipre, Kaspersky or MS SE, etc 2GB-3GB of RAM should be enough.
A nice video card might also be worth it if you plan to use your computer to watch Blu-rays, and play a few games.