Memory Bandwidth vs. CL?

Currently my Dell Inspiron 6100 (Core2Duo T7200 with Intel 945P chipset) has 1gig of PC2-4200 CL 4 but I was thinking of upgrading to 2gigs. I was wondering: Is it better to get RAM with lower bandwidth but faster CL or faster bandwidth but slower CL? I was looking at PC2-4200 CL 4 vs. PC2-5300 CL 5 and they both cost about the same. Both are supported by my laptop. Wondering what would be the better idea.

 

PC4200 runs at 533 Mhz, whereas PC5300 runs at 667 Mhz.

There is literally no performance difference between these two speeds. The reason is because the CAS latency of the PC5300 is higher. Thus, while the PC5300 RAM can handle higher memory bandwidths, it is limited by the number of cycles it can perform per second. Think of the highway analogy - usually it's specific to the amount of RAM, but I'll modify it slightly -

RAM is like a highway. The faster the RAM, the faster the cars zip along the highway. However, if you increase the CAS latency, you constrict the number of lanes of traffic that the highway can handle. So, while the cars will still be zipping along at 60 mph on the PC5300 highway, they will be limited to 4 lanes of traffic, while on the PC4200 highway, there will be a tradeoff between lower speeds, but more lanes of traffic.

If both are supported by your laptop, I'd recommend getting the 667 Mhz (PC5300) RAM modules. You never know when someday the extra memory bandwidth might become useful.

 

You want the 667MHz RAM, if you can afford it. CAS latency of the 533 is one "tick" slower than the CAS latency of the 667MHz RAM, but that just means that they run at the EXACT same latency in wall-clock time. So 667 just transfers blocks of data faster, while not losing any performance when randomly looking up data.