Not necessarily. Here's an example: I own a Gigabyte 7VRXP motherboard that uses the KT333 Via Chipset. This mobo supports 333Mhz Ram modules but it only support 266Mhz processors (mobo's FSB is 133 but the memory bus is 166 <- Multiply these by 2). That means that I can't run a 333FSB processor (actually I can but it won't run at it's top speed, mobo multiplier times the FSB).
Here's the catch although I have PC2700 Ram running it at that speed would mean my CPU and my Ram would run asynchronously (one at 266 and the other at 333) which could cause latency (processor being swamped by memory and then having to wait a few cycles before accessing Ram again). The solution was to set my memory to run at 266 but up the memory timings (select High Performance in the Bios). Even though I'm running my Ram at a slower "speed" I gain a little performance by running synchronously and having aggressive memory timings.
Before you buy a new CPU, check on your motherboard's manufacturer's website to confirm what CPU FSB it will support.
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