I use a Seagate Barracuda 500GB as my boot drive and I did a test today to see how fast it booted because it seemed fast compared to most other PCs. It booted to Windows 8 screen in 26.56 seconds. Based of some videos and benchmark comparisons this is at least 20 seconds faster than the average HDD, 30-40 seconds faster than some. I have no idea how its booting so fast on just a regular hard drive. My question is with this type of speed would an SSD really be worth it?
Windows 8 boots very fast due to a kind of hybrid shutdown/hibernate. The Win8 specialists here can explain the details.
Everything else that loads will be subject to that slow drive's access time and read data rate. An SSD is orders of magnitude faster so you will benefit in all aspects of your computing where you are loading or writing data.
When I was building my brothers rig, ended up going with a 120GB 840 and Windows 8. I'm used to SSD speeds with my Vertex 4 SSD and Win7 daily rig, but I was surprised by how fast it was.
Though that was a clean install.
Go for the Samsung 840, if you intend to us it as a plain boot and programs drive it will serve you fine.