You dont need a 4890 to play WOW. A 4670 would suffice, or wait for a 4770. Crysis is another story all together and any single gpu will choke on it with high presets.
Your cpu is a bit weak.. may not be worth the price of the 4890 given the systems weaknesses. Get a more value based card, I would even say to wait till the 4770. As for size, the 4890 is about 9" long.
Also, the 4890 needs a very good PSU, what do you have?