what kind of memory are you looking at? I imagine you're looking at DDR2-800 or something. at those speeds, the difference between C4 and C5 is negligible, especially on the Intel platform (it matters more with AM2 because of the low-latency memory controller). on my e6400 platform, i got substantially faster results from running my memory at 963MHz @ 5-4-5-10 than i did at 800MHz @4-4-4-10, even though i was using a memory divider. intel likes bandwidth more than it likes low latency.
all things being equal though, get the lowest latency memory that fits in your price range. the nice thing is that lower latency memory usually overclocks well if you loosen the timings, e.g. DDR2-800 @4-4-4-10 might run fine at DDR2-1000 @5-5-5-12.