There was high excitement at the leading US mail order personal computer houses as un-official word came from within IBM Corp yesterday that the substance of a story in the Wall Street Journal that it was in talks to buy a stake in or more likely to acquire Northgate Computer Systems Inc, an Eden Prairie, Minnesota personal computer builder that outgrew its strength in 1990 and had to retrench. The idea is for IBM to derive volume from the mass market personal computer business without sullying the IBM name or that of its PS/2 line, which presumably becomes a highermargin business for volume endusers that buy direct from IBM only. The excitement is caused by the expectatation that the likes of Digital Equipment Corp and Unisys Corp will feel that they have to have a mailorder house too DEC is already building its own mail order operation but may now think it needs to do more. Names doing the rounds include Dell Computer Corp, CompuAdd Corp and even Compaq Computer Corp. Northgate made profit of $2.7m on sales of $167m last year.