As best I can tell, the best bang for your buck is to mine primecoins with your CPU and to mine Digitalcoins with your GPUs. Now, if a GPU miner for XPMs ever comes out, then they will be in competition to the detriment of both DGC and LTC. However, I am starting do doubt whether a GPU miner which is much more efficient than the current CPU miners is likely to be created.
This is completely arbitrary. First, this isn't true merged mining (as stated before), secondly (and more importantly), the profitability of scrypt-coins fluctuates wildly. Because of profitability-comparison-sites such as coinchoose. com and multi-coin mining pools that switch coin depending on profitability, you see the most profitable coin being mined extensively until the difficulty spike pushes the profitability way down, after which it takes time to recover and the cycle repeats.
