Unit sales of personal computers through retail and mail order rose 21% in April, though revenue slipped as prices fell and sales of low-cost PCs with Intel Corp.'s Celeron chip surged, a report said. Revenue fell 2.2% in April from a year earlier, and the average sales price fell 19% to $928, according to PC Data Corp. Sales of computers costing less than $1,000 almost doubled, making up 71% of all retail sales. The report suggests that PC makers will face increased profit pressure as buyers flock to cheaper systems rather than PCs with all the bells and whistles. "All the growth in the market was in sub-$1,000 PCs," said analyst Stephen Baker of PC Data, a market researcher in Reston, Va. "The average selling price is down a lot more than expected." Analysts have said that if the number of Celeron chips sold is higher than expected and sales of Pentium III are lower, Intel's second-quarter earnings could suffer.