DDR2 has finally taken over as the most popular memory chip used in PCs, after a much longer than expected delay.

The crossover to the second generation of double data rate memory from the previous mainstream chip, DDR-400 (double data rate, 400 megahertz), took place this week on both the spot DRAM market and the contract market, industry researcher DRAMeXchange said.

It's been a long time coming. The chips entered the market far later than expected, mainly due to heat problems. In fact, DDR-400 , which has reigned as industry leader for the past few years, was supposed to be a stop-gap chip with such a short life span that some companies hadn't even planned to support it with a chipset.

But when DDR-400 showed up on Intel's product road map in late 2002, component makers began to use it, and put off developing compatible parts for DDR2. Intel microprocessors power around 80 percent of all PCs, making an Intel stamp of approval vital to computer industry parts makers.

AMD helped further put off demand for DDR2 this year. The company waited until the middle of the year to launch its first desktop and laptop processors that support DDR2 because it didn't think initial versions of the chip added substantial performance benefits to AMD processor-based systems, yet commanded a higher price.

The first DDR2 chips ran at 400MHz and then 533MHz, but AMD waited until DDR2 chips running at 667MHz reached mass production levels, which lowered price and ensured a plentiful supply. Currently, a wide range of Intel and AMD systems support DDR2.

DRAMeXchange noted Wednesday that prices may become an issue again for DDR2. Prices for the chips rose 8.8 percent last week, compared to the previous week, the company said, due to strong demand for PCs. The market researcher expects prices to remain strong throughout September, the traditional time when back-to-school PC buying increases buying activity in the DRAM market.