With WOTLK you definitely want 8GB of ram. You can have the fastest CPU in the world and it'll still be sluggish when you're low on ram.

Pri 1 - 64bit OS & 8GB Ram (You don't want to run 2008 Enterprise 32bit (which supports enough ram, but has other drawbacks)).
Pri 2 - Quad-core.
Pri 3 - Gfx card

That's how i would prioritize.