even if it is slower, more memory is better since you'll page less often and more items will be available in cache.
if you have a few hours and like to tinker, see if your existing ram can run at ddr2-667. first, download burn-in software from here: http://www.passmark.com/products/bit.htm
next, go into your bios and configure your existing ram to run at ddr2-667. boot up windows and run passmark to stress your ram.
if it "passes", you can add the corsair at full speed. if not, reset your bios and let the bios run it at the speed it sees fit.
I just bought 2 gigs pc-5400 (667 mghz) high performance Corsair memory to install in my system which is currently running with 1 gig pc-4200 (533 mghz) memory. Would I get better system performance by adding the two gigs for a total of 3 gigs running at 533 mghz or by replacing the existing memory with the 2 gigs running at 667 mghz?