I personally always prefer the clean install route rather than the upgrade route wherever possible.
Reason for this is that you end up with a vanilla Windows install, ready to set up the way you like and also to review what 3rd party programs you bring back on board.
That must be a 64-bit system you have there (32-bit systems can only utilise 4GB RAM) but are you running 32-bit Windows or 64-bit?
If 32-bit, then definitely download the ISO for 64-bit Windows 10 and clean install it (you cannot change architectures in an upgrade, this is technically not possible).
You should be able to download the ISO for 64-bit Windows 10 and clean install it, overwriting what you already have.
But be aware that if you do this, you definitely will not be able to revert to your earlier OS (Microsoft gives you 30 days to do this by preserving your original OS on your hard drive). Also, be sure to back everything up before you begin as reformatting and reinstalling your OS *will* cause you to lose all your data.
Is your graphics integrated? This means it will share system memory, meaning less is available to Windows. Less memory means less performance. But with 8GB and a 64-bit OS, you should not have a problem.