Rezabrya, keep in mind that your new laptop has TWO memory slots/sockets/connectors, which each are capable of holding ONE memory module, or DIMM.
You didn't pay extra for the memory because it was 1333 MHz memory. You paid extra because your 4GB of total memory consisted of ONE 4GB DIMM instead of TWO 2GB DIMMs. In fact, if you look at Lenovo's current tabook, you'll see that ALL 4GB DIMMs that currently ship with the T510/W510 are 1333 MHz memory, and ALL other DIMMs that currently ship with those two machines are 1066 MHz memory. Also, if you look at the web page at which you configure the T510 laptop for purchase, you'll see that for all of the 1333 MHz memory, it says "at 1066 MHz," meaning that the memory, despite being 1333 MHz memory, will only be run by the machine at 1066 MHz.
Also, because you bought your memory as just ONE DIMM instead of as TWO DIMMs, you will not receive the speed advantages of "dual-channel memory access" (where the computer accesses two DIMMs simultaneously to improve the speed of memory access). Unfortunately, the "Help me decide" link for memory in the purchase configuration page does NOT discuss "dual-channel memory access."
Note that the main reason someone would order just a single 4GB DIMM with their laptop, is if they plan to order another matching 4GB DIMM later to get a total of 8GB of memory. Otherwise, if you order two 2GB DIMMs now, and later want 8GB total, then you'll have to dump the two 2GB DIMMs and replace them with two 4GB DIMMs, which can be a more costly route to 8GB. The downside of that cost-saving strategy of buying just one DIMM now, of course, is that you forego the speed advantage of dual-channel memory access until you buy and install the second DIMM.
Finally, there seems to be a misconception by users that 1333 MHz memory costs more than 1066 MHz memory. This is not necessarily the case. For example, if you go to the highly-regarded vendor crucial.com, and look up what memory they sell for the T510, you'll see that 1333 MHz memory and 1066 MHz memory for that machine, as with many other machines, cost exactly the same.
So if you're not familiar with the in's and out's of all of this, and don't read the options in the configuration page carefully, you can easily make mistakes. But in your case it's certainly not the end of the world, whichever way you decide to go. For dual-channel memory access, though, it's preferable that BOTH DIMMs are of the same type from the same manufacturer. So if you simply go to crucial.com and pay whatever meager amount they're charging for another 4GB DIMM, I'm not certain that it will have the same TIMING specs as your current 4GB DIMM, which may or may not make any noticeable performance difference (probably not, is my guess, although I can't guarantee that). Or, if you're so inclined, you can buy a second 4GB DIMM from Lenovo, for whatever they charge, and as long as you don't wait so long that the DIMM technology changes, it should match what's already coming with the machine.
In any case, good luck!
