I went from a Q6600 (stock speed), 8GB of good DDR2 (800mhz), and a 4870 X2 video card.

I upgraded to an i7 920 (stock again), 12GB of DDR3 (1600mhz), on an Asus P6T motherboard.
Unfortunately, the board did not like my 4870X2 Ati card, but I downgraded to a GTX 275 (comparable to the 4870 X1).



With the Q6600...
- I could break follow, mounted in Dalaran.
- I could break follow, flying with quick altitude changes on epic mounts (any toon leading).
- I could lose mounted follow in Alterac Valley.
- The game was playable anywhere, but my CPU would reach 100% in Dalaran/Wintergrasp.

With the i7 920...
- I cannot break follow in Dalaran, unless they get caught on a corner.
- I can only break follow (epic flying) if the Paladin is leading with Crusader Aura on (not including crossing phased areas).
- Alterac Valley is smooth everywhere.
- My CPU does not go over 25% anywhere.
- Dalaran is not high frames per second, but even very expensive computers aren't 60 FPS everywhere in Dal.



The i7 920 was initially purchased shortly after moving to IS Boxer from Keyclone.
IS Boxer is a bit more intensive on the system, so if I was still using KC the improvement would likely be slightly better then it is; and it is a massive improvement.

I've also put warcraft on a 64GB SSD, which was also a huge improvement.
But that was quite a bit after the CPU/Ram/Motherboard upgrade and the video downgrade.