First it probably needs to be said that you cannot upgrade from 64bit architecture to 32bit architecture or vice versa without performing a custom clean install.
Second with respect to online gaming, I would think that, that would be more dependent on your hardware (processor CPU, memory, Video GPU, etc.,) more so than the operating system.
And third, there are many resources where you can obtain an ISO for Windows 7 Link Removed
But you will need to purchase a legitimate and legal product key from a reputable source in order to install it without issue. Link Removed