You cannot change to 64 bits without a complete clean install of windows 8.1 64 bits. More memory doesn't increase speed or solve problems .Look somewhere else to solve that problem. If you have a cpu capable of running 64 bits--not all vostros can run that-- and you have a dell windows 8 64 bits disk, boot to it and do a Custom install. If you did boot to the install disk and got that error, you cannot run 64 bits. See if you can return the extra memory or sell it. To check to see if you can run 64 bits, open the System page in Control Panel and see what it says.You are way behind the times by not installing 8.1 free update. You could get a free update to windows 10 32 bits after July 29 if you install 8.1 update and your computer is compatible. 
Dell Vostro I am running Windows 8.0 Pro 34 bit on the machine, have upgraded memory to 8GB, however have been told that I need to run Windows 8.0  64 bit otherwise OS system will not use the 8GB. Have tried installing the 64 bit version , but I get message "this app wont run on your PC", what can I do?  Reason for installing additional memory is due to machine slowing down, and SupportAssist states that maximum 8GB memory is supported. By the way I have had the computer for over 4 years and have been very happy with the performance.
Dell Vostro 3700