Just fyi - an E6600 is £130, a Q6600 is £185 (for a G0), an E6700, £170, Q6700, £331, E6400, £105. Q6400 £119 delivered.
Ok, there is no Q6400, but the Xeon X2310 is 2.13 GHz, Intel 2 Duo architecture, Quad Core and was also going to be released as a Q6400, but only the ES' seem to exist. Works in "ordinary" Socket 775 boards which take Quads, too!
/me just bought one :P
Either way i'd like to point out that either you go "super cheap" with an E2160 (£58, does 3.2 GHz+ with a decent mobo/air cooling), get this lovely entry Quad (probably "Only" does 3 GHz, maybe even less), or you plump the cash for a Q6600 G0 - anything in the middle is imo a little pointless.