The Avro RJ-X is basically the old BAe146/Avro RJ design. The RJ-X will mainly have new engines, the Allied Signal AS900 if I´m right which did its maiden flight some days ago. Second the fuselage will be lighter, more composite parts will be added instead of metal. All in all BAe promises a cost reduction compared to the current RJ70/85/100 of about 15%. So far only Eurowings has expressed interested in this type, for 6 frames. Other possibly interested airlines are Aer Lingus, British Regional and City Flyer Express.
I doubt that the RJ-X will be ever launched as there are cheaper planes in the same category, the 728JET/ERJ-170 and the CRJ700.