Speed matters up to a point, as in DDR500 is faster than DDR333 by a good amount that you can see in real use.
the difference from 215 and 240 would be marginal in comparrison to going from 1 to 2 gigs. Faster memory does have its merits though. It allows you to run higher FSB's than you normally could(obviously meaning a faster system overall). Instead of looking for the fastest RAM you can afford and hoping that it will make a difference, decide on how high you want to run your FSB, multiply it with the available RAM multipliers and you will see your options. Go for the fastest speed you can practically afford.
I can already feel the heated arguments starting, But I have always benifitted more from raw speed than marginally better timings. It's a different ball game when you are talking about large differences in timings though...
