There are duds in every brand and model. I haven't come across dud memory, but I have had at least three defective optical drives, two Sonys and a Samsung. With the latter, smoke came out of the drive as soon as I power on the Dimension 2350 it was installed in; I RMA'ed it to Newegg and received a like unit, which has functioned perfectly. The occasional RMA is par for the course in DIY hardware upgrades.
Regarding the memory, if it doesn't work, you can RMA it back to Newegg. It also has a lifetime guarantee from GSKILL, which means they'll look after you if you have problems in the near to mid future. If you're using the 3100 for Web surfing and other light uses, you should be able to get several more years use out of it with a few mods. Regretfully, a $40 Window 8 upgrade isn't one of them, and XP support will be ended next year.