For quad boxing, I think 4GB of ram is imperative. I'd wager you'd see better performance out of a dual-core and 4GB of ram than a quad-core and 3GB of ram.

A 4GB, Q6600, 8800-series system is a very capable 4xClient system. But I have 4xboxed for 5 months now with a 4GB, E6600 dual-core, 7600 series card with almost no issues other than crowded Shattrath and AV when there's 40 people nearby.