hehe, well it depends how fussy you want to be
That link for example is just user benchmarks so I don't consider it much of anything personally. Second, simply because something looks a lot slower in benchmarks doesn't mean it will be on your computer;
you are doing the wrong comparison It should be a HDD vs SSD.
http://www.anandtech.com/show/7864/crucial-m550-review-128gb-256gb-512gb-and-1tb-models-tested/5
yes, it does look a bit slower, but the concern wasn't the SSD speed, the concern was updating an old computer. I believe you are just worried about having an experience similar to that of your current HDD, which is expected!
If you compare any SSD to your current HDD you will be more than amazed and satisfied with what you get.
But in the real user settings, those benchmarks aren't so accurate. You would probably not notice the speed difference between the m500 and the m550, all the while the m500 is $50 less ; take into consideration that you wanted to get a ram upgrade to.
I'm not trying to get you into buyer's remorse mode, I'm just saying to not get into the consumer worry about buying something bad. I see some many people on here trying to build $1,500 computer for games that $700-800 pcs could play at max settings, or they expect them to last 10 years with decent performance...(think of the difference from a computer from 2006 compared to now) and for the most part it's all just marketing crap to get you to buy something more expensive.
As an example, my 120gb Patriot Fyre that is 3 years old probably is not very fast, most likely worse than the m500, but it boots my computer up in 10-15 seconds and opens up applications pretty much instantly or within 1 second. This is 300%+ faster than the HDD I had before it.
The mx100 would have been better than the m500 at it's current price, but it's not much faster either.
http://www.anandtech.com/show/8066/crucial-mx100-256gb-512gb-review/9
So all in all, just be happy with your 840 EVO. Worst case scenario again being that you can just buy a $70 Crucial/Sandisk 256gb SSD next year to replace in your laptop and put your 840 in the new build you plan to do.
Enjoy the speed!