If you get a full cover water block, the VRM and whatever that small chip in the front is, will be covered. If you go for a universal water block, then you'll need to find heatsinks or other methods of cooling, though will not be as effective as a full cover.
How cold do you want to go? With the OC-ed CPU, you should be hitting around 55 - 60 degrees on a 240 rad. Plugging in the 290 and OC-ing that, expect to see temperatures of possibly about 65-75 (guessing here) on both your CPU and GPU if running both on 240mm.

IMHO, throw in at the very least a 360 rad. If you already have the 240, get another 240/360. skip a 120mm.