I DID mention in my previous message that the memory speed is limited by the maximum frequencies supported by both the CPU and motherboard. Unfortunately it seems that your CPU only supports PC3-8500 (DDR3-1066 MHz) maximum memory frequency, so any memory used with it will downgrade to PC3-8500 anyway. So in fact the RAM stick that you already had is a PC3-10600 (DDR3-1333) as well (at least according to the PC info on newegg), just that it works at a lower speed because of the CPU, so it appears as a PC-8500 to system information software.
From my online findings, your CPU socket on the motherboard is AM3 (not "AM3+"). If you happen to replace that (otherwise pretty sweet, at least a couple of years ago it was) dual-core (X2) CPU with one with more bandwidth (Front Speed Bus frequency), the RAM sticks will work at their full potential (unless the motherboard's memory speed limit is also 1066 MHz, which is probably not the case). Such CPUs for your socket are any X3 - 450/455/460 (those are triple core) or one of the quad cores X4 640 / 645 - the higher the model numbers, the higher their frequency - if you can still find those (maybe on ebay, I don't know). Otherwise, from what I could tell from online benchmarks a couple of years ago, there isn't much performance benefit between 1066 and 1333 MHz RAM anyway.
The reason the Crucial app/website (and greg) initially pointed you to a faster RAM is probably that 1066 MHz RAM sticks are no longer on sale, most probably, so they gave you a compatible one instead.
Besides being of same capacity, for two memory sticks to work in dual-channel it's also recommended that they are as similar as possible (have the same latency timings, be from the same manufacturer, and preferably even the same factory lot) but that's mostly commercial mambo-jumbo in my opinion and even different latency RAM sticks should work together. However, you can at least rest easy you didn't spend the money on the second stick completely uselessly and maybe it could also be considered an excuse for the Crucial employees to have recommended that buy.
Speaking of Speccy, you can verify the temperatures of some components with it, so for example you current CPU's maximum temperature ceiling is 74 degrees Celsius. You can check that when the PC is worked hard, and if it ever reaches 75 for prolonged times (say, more than a minute or two at most), you know you need to worry about overheating. I thought I'd share that with you since I stumbled on the information online.
Another thing in Speccy related to your PC, if the "Graphics" is recognized as "Radeon (HD) 3000" (which is the default video chip for your PC, integrated into the motherboard), then you probably don't already have a dedicated video card, and that integrated chip is only good for dual monitor configurations. But it's definitely good for that at least, according to the AMD website.