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#11
1.35 voltage memory is optimized for low battery usage, the 1.5 is not (not a huge hit for a laptop with your specs). Performance is theoretically worse but it's hard to notice.
Your laptop should take both as standard is 1.5 Volt. If you notice, Crucial recommends some RAM banks that need 1.5 V as well for your laptop.
Unless you have a very good reason to do so (heavy music/video editing, virtual machines) you should be fine with 8 GB, you won't notice a lot of difference going up to 16 GB.
Although you may want to purchase them now as when you may need them (in the future when you want to upgrade) it will be harder and more pricey to get a hold on them.
Changing RAM is more or less model-independent unless they placed the ram slots in funky ways (if it isn't a netbook it shoud be fine), you open a panel under it (that should have a symbol looking like a RAM bank close to it), and operate the same slot SODIMM fastening springs to remove/add banks. Watch a video and you know how to do it on your own laptop as well.