Don't invest into upgrades, ever. The reason is if you ever need to reinstall your OS, you need to activate the original OS after installing it first, doing every update possible, and then running the upgrade disk to get the OS patched, and then activate Windows 8 and do all the upgrades for that. It takes as long as three times the rate to do this compared to installing an OEM copy of Windows 8, activating it, and doing the updates for it, then turning around to sell your Windows 7 key and making back some money on that.
Usually, you can get it for as little as $90 according to Google Shopping at this point. Windows 7, on the other hand, sells for as cheap as $70. Now...the big question I need to ask you is 'Why do you want Windows 8 over Windows 7'?
Because if you have never, ever used Windows 8 before, you need to be prepared for a couple of things:
1) It does run more efficiently on older PCs than Vista/7 does, mainly because it doesn't use up as much in resources (this same OS runs on tablets and smartphones, so let that give you an idea of how different it is with performance).
2) Because the OS is capable of running on tablets and smartphones, the GUI is vastly different. With traditional copies of Windows, you had a certain 'image' of the desktop that was commonplace and people were used to (Start button/menu, My Computer link, etc.). Windows 8 changes that with its implementation of Start Screen, which is a display that encompasses 'app-style' bubbles for you to click and run. There is no My Computer icon unless you really dig; however, doing things like "C:\", "D:\" in the Search bar on the Start Screen does allow you to access your Hard Drives, but unless you know how to navigate them for content, it can be very confusing.
3) Relevant to some of the things I said in number 2, navigating Windows 8 is vastly more difficult for simple PC users. The absence of Start Menu, folders for installed programs consolidated down to simple .exe's do make it where if a folder contains more than one application for you to run, the default application's .exe gets added into Start Screen but not any others affiliated with it. If you need to do any immediate diagnostic work on Windows 8, you NEED to boot into Windows 8, search for Advanced Startup, and tell the PC to reboot and allow for the choices to do things as boot into Safe Mode (with Windows 7 and back, you could hit F8 on bootup to Windows and go instantly to the choices; not with Windows 8).
If you need a copy of Windows, stick with Windows 7. It's support is sure to be guaranteed through 2020 since you can usually get a full decade out of a real solid OS (and so many people have jumped on the 7 bandwagon more coming from 8 as of late because of the reasons above and then some); And make sure you get 64-Bit. Any PC manufactured from mid-2000's onward is most likely a 64-bit CPU and can take the upgrade; plus, 32-Bit versions are held back by only using 3-3.5 GB of RAM while 64-Bit's can go much higher.
For the TL;DR enthusiasts, stick with Windows 7 for now.
