The best replacement for 667mhz is 800mhz ram.

The best alternatives to 667mhz are rated as following:
★★★★★  800mhz ram
★★★★★  533mhz
★★★★  1066mhz
★★★★  800mhz
★★  ddr2 800mhz
★★  800 mhz ram
★★  800mhz ddr2
★  800 mhz
★  2gb 800mhz
  333mhz
  ddr2 533mhz
  4gb 800mhz

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Speaking about 667mhz, one probably shouldn't forget about 800mhz.

Other search terms related to 667mhz and their ratings are:
★★★★★ 800mhz
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667mhz
800mhz ram
533mhz