Just bought a T420s to arrive soon, with i5/2520 processor, integrated graphics, 128GB SSD, and 4GB RAM. I want to add a second stick of 4GB RAM and am looking for advice about the best type to get. I just need something equivalent to what is probably coming in the machine, with a reasonable price. Is something like this suitable:
Patriot Signature 4 GB PC3-10600 (1333 MHz) DDR3 SODIMM Notebook Memory PSD34G13332S
or should I be looking for:
Samsung MV-3T4G4 4GB DDR3 Laptop SDRAM (1333MHz PC3-10600)

Or do you have exact product numbers that are best for the machine?

The late model Lenovo's that I have seen come with Elipda memory. I have not been able to see a difference between Kingston, PNY, Corsair, Crucial, or Centon sticks. Every stick that I have tested in all the different brands have given the exact same performance. Go with the 1333MHz memory that has the best price.

It's best to use matching RAM sticks (same make/model) for compatibility. If the sticks have different timings/voltages it will cause instability problems, and an 8GB kit goes for $30-$40. In fact you could get a 16GB kit for around $100.

A 16GB kit for $100 would be a phenomenal deal at this point in time (Feb 2012). Prices are coming down, but the best that I have them at is around $150-160.

The computer is supposed to be delivered today. I think I will look at what chip is in it now, and see if I can match it, or whether it is just the same expense to buy a 8GB kit.

You must be pretty excited. I know I would be stoked.

It will be difficult to find an exact match, as manufacturer's will use the same part number for different batches. I have seen 1R modules and 2R modules with the exact same mfg model # and UPC.

If you are really concerned about it, I would go ahead and plan on an 8GB kit.

Received the Thinkpad and have it installed now. I'm quite pleased, especially the keyboard, size and weight. The RAM is Samsung and it sells for about $30 for another stick, or I can get some Crucial or PNY or Kingston 2 x 4GB for about $40.

So, is it okay to add a second stick of Corsair or Patriot or Crucial to go with this Samsung? I haven't had problems before doing that, but it usually has been on Macs.

I bought my T520 with 8Gb RAM, original factory-supplied samsung 4Gb RAM and patriot 4Gb RAM added by the reseller - works just fine I don't think the manufacturer's name makes any difference, as long as the RAM parameters are matching

I went ahead and ordered a matched pair of Crucial memory chips. An additional Samsung chip sells for about $30, and it wasn't too much more to purchase a matched pair of Crucial. Once I see that the RAM is working okay, I will sell the Samsung chip online.