3yr old ASUS - need new GPU - help! (compatibility)

I am in a bit of a situation here where i need to buy a new rig, but it may be a few months and i desparately need a performance increase from my old GeForce 6800 ultra GPU... playing Warhammer Online at 12 - 15 constant FPS (using lowest possible settings in game) is killing me guys.

So, here is what i was going to do:

Buy the new GPU, use it with my current old rig, and wait a few months to buy all the rest of my new setup.

(Question 1: are the new nVidia (or ATI) GPU's compatible with a 3 year old ASUS mobo?) [i will post the exact model when i get home from college]

{if not, what is a cheap temporary GPU solution that IS compatible}


Buy a new case and move my parts from the alienware case to a new case (its a 3 year old alienware rig with some heating issues)

(Question 2: are alienware setups built any different to where i shouldn't try to transfer cases, or is it just a fancy looking over-priced case/brand with parts that i could have spend half the amount on; and is easily transferable from case to case)


edit: new question, it IS the outdated GPU that is causing the horrible framerate, right? because im also running with a 3.2 ghz p4 processor...