Underlined/Bolded: These are the various capacities of RAM modules, or DIMM's, you can use. The specifications are not stating what type to use if ADDING more RAM, but what types to use according to the SIZE of the modules.

2 GB PC2-4200 DDR2 SDRAM 533 MHz SO DIMM:
This is the type you should use if you are going to use 2GB sticks (or Modules) of RAM. Two modules would give you 4GB of RAM which is the maximum your computer can support.

or (that was the key word in that sentence )

256 MB, 512 MB, and 1 GB PC2-5300 DDR2 SDRAM 667 MHz SO DIMM:
This is the type you should use if you are going to use any of these capacities. If you have 4 slots you could use 4/1GB sticks of this type to get 4GB total.

Do NOT mix the two types. Go with one or the other. If you are trying to add more RAM you need to find out what type are already in there. IMO if you want 4GB total you'd be better off with 4 DIMMs of the PC2-5300 because they are faster at 667 Mhz but your motherboard has to have 4 slots to put them in.

What operating system are you using? Win XP 32-bit won't recognize more than 3.1 or 3.2 GB of memory.

You asked: "Also, is there anything wrong with having say 2.5gb of ram (ie. not round number)?"
Matched pairs of the exact same brand, type and capacity are usually best but, for example, if you have 4 slots you can sometimes put say two 1 gig sticks in one set of slots and two 256's in the other two. They still need to be the same type.

Brand? Personally I prefer Corsair. They have good tech support too. Kinston is good but when I built the computer I'm typing this on I was also looking at Crucial. Some people had reported problems with their memory and the ASUS motherboard I got so I decided against them.

Hope that helps,
Ish

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I always get Kingston.

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Thanks for the replies. Why would they make such conditions for using 2gb sticks?

What if I'm going for 3gb? There's already a 1gb of the faster kind in there. I want to add a 2gb stick. One the one hand they say use the slower type if adding a 2gb stick, on the hand you're not supposed to mix.

You haven't mentioned what OS you're using. That could make a difference on how much RAM you might need or be able to use.

On the 2GB sticks: It's probably a stability issue on that particular computer when using the larger capacity RAM modules. Whatever the case, the people that built it have tested it and that's why they recommended what they did.

You could add a 2GB stick and it might work OK, or it might not. If it does work the faster 1GB stick will be limited to run at the slower RAM's speed.

If you're running Win Xp I'd just add another 1GB of the faster stuff and be done with it. XP will run with 1GB but works better with two. That's plenty for XP in about all but the most extreme situations.

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I think maybe 2gb PC2-5300 667 sticks weren't out when the laptop was released? Either that or it's a typo. Lenovo now shows a p/n for such a stick along with the slower one. Either way, I looked through the thinkpad forum and people seem to be using the faster 2gb without any issues.

Thanks for your help! I ended up getting the Corsair version of this. It was marginally cheaper and a ton of positive reviews on newegg.

Excellent, I hope it works out OK.

If you do have a problem try using just the 2GB Corsair stick by itself. There could possibly be compatability issues with the existing RAM and the new Corsair stick. That's why I said it's best to use the same brand and type.

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'05 X3 3.0i Alpine White/Gray Leather/Alum. Trim
Sport/Steptronic/Servotronic/Panoramic
Prem. Sound/Staggered 18" Style 114 Y-Spokes