Older versions of RealPlayer are not disabled, though newer content will lead to repeated prompting for you to upgrade.
So long as you have sufficient real RAM (64 Mb, with VM turned on and set above 96 Mb) there's no reason not to upgrade from G2 or 7 to RealPlayer 8. Playback performance of video on slower machines may be disappointing, but the application should run properly and no worse than earlier versions.
Before installing the new version, it's a good idea to remove the files of the older version (by deleting the Real directories from the Extensions and Application Support folders and deleting Real files in the Preferences folder.)