Speeds will increase for PCs next week when Advanced Micro Devices releases a 700-MHz version of its Athlon chip, a move that will be followed by new computers from IBM and Compaq Computer.  The new chip, expected to be announced Monday, will mean that AMD will continue to enjoy a speed and performance advantage over Intel's top chips, according to analysts and other sources. Intel won't be far behind, however; it is slated to come out with 700-MHz and 733-MHz Pentium IIIs on October 25, sources said. The fastest Pentium III now tops out at 600 MHz. Overall, Athlon achieves a higher level of performance than the Pentium III at equal speeds, leading to crisper, more realistic graphics, according to testers.