All World of Warcraft accounts have been automatically updated to include TBC, WotLK and Cataclysm. So yes basically should be able to play the game without any problems, however you will not be able to go past level 85 and as you've already mentioned: the graphics requirements are higher.
Now this doesn't mean that it's unplayable for you. If you just want to play the game and you don't mind low graphics you can just put the settings on the lowest available. The graphical requirements are noticable when playing on ultra. On low settings it's marginal compared to the older expansions.
When it comes to playing Old content you might not be too lucky. The old Azeroth doesn't exist anymore and there are very few zones that still are the same or similar to what they looked like before the Cataclysm. Northrend and Outland are still the same with minor differences but the character development (skill-trees and abilities in genreal) has changed.