If the Corsair XMS2 DHX TwinX DDR2 PC6400/800MHz CL5 2x1GB is already in the computer, should I just buy another pair of those or would it be better if I went with XMS2 DHX TwinX DDR2 PC6400/800MHz CL4 2x1GB?

The price difference between those is negligible over here but still noticeable so I want to know if the CL4 timing makes a difference when paired with CL5 sticks.

You need to match the CAS Latency, go with the CL5.

You could try tightening the timings on what you have to CL4 and checking with memtest86+ or Prime95 for errors. If tightening the timings works, you can safely get the CL4.

Those particular Corsairs are not so flexible, but give it a shot. Got a real good motherboard?

I'm buying a used computer so I can't test the memory yet, you'll probably diss me out on this but I found one that's cheaper than building myself and I'm using an escrow service so I haven't actually paid the guy yet (and I can send it back if it doesn't live up to the claimed specs). It's most likely arriving tomorrow.

However, I want to order the ram today so I don't have to wait too long for that.

Can I go wrong with CL4 timings? I'll probably settle with them if it means that can wait one day less. The only problem would be if they're not compatible with CL5 timings but I should be able to downgrade, right? The reason I'm not sure about getting CL5 is because the timings wasn't specified but I'll ask him about it now in case he knows.

Actually, never mind, sorry! I didn't see that he put CL4 on a new line of text :S

Sorry again!

What kind of motherboard, precisely, it it allows you to adjust the RAM timings you should be able to make the CL4 work, if it doesn't work at CL4, you can change the timings to CL5...if the motherboard allows for such adjustment.

Get CL5 to be safe. We don't want to screw you over. The performance difference is sorry and invisible.

Agreed.

Wait, should I get CL5 sticks even though the other ones are CL4? Seems odd to me. The motherboard is an Asus P5B.

I say wait untill you get your computer before making a purchase.

Do you know what components are inside the PC?

Your first word is "IF".

This P5B only has 2 Memory slots:
http://www.newegg.com/Product/Prod [...] 6813131186

blargher, come man. We're telling you to MATCH the CL of what you already have (if you do indeed have room to add 2 more sticks). Sheesh. First you said it was CL5, then you said you found out it was CL4. Now its a P5B that only has 2 slots but you want to ADD 2 more sticks.... I have to agree with the "I say wait untill you get your computer before making a purchase." When you KNOW what you have (because you've seen it with your own eyes) worry about upgrading then.