I only played HL2:EP2 on a single core CPU and can confirm it plays smoothly with my 7800GT graphics card, so it is most likely your graphics card causing the issues. What card do you have? I can also confirm the HL2 runs perfectly well on the older Radeon 9800 Pro cards at 1024x768 as that is what I had when it came out.

The one criticism of EP2 is that it was more linear than promised, but the original HL2 was very linear and no one complained much about that. When a game is story driven as HL is you can't make it too open and non-linear because the player could then end up bypassing some of the story completely and trigger an event past their current progression within the game.

One of the new opponents is also a real asshole and you will learn to hate it with a passion.
Their AI is definitely a surprise compared to what we are used to. It is great how they can track you and even work out where you are heading so they can flank you and give you a nice surprise when you reach your destination, only to discover they are already there waiting for you. No longer is it safe to hide behind a wall and wait, as they will be moving around behind you while you sit there.