If you're already planning to watercool your CPU then adding the GPU in the loop would make sense providing you had enough radiator to handle the extra heat.
If you only wanted to watercool the graphics card then a high end after-market air cooler would do the same job and be alot cheaper.
I think watercooling is only really worth the cost if you want a very quiet computer and you also want to overclock, otherwise an after-market air heatsink either with the fans on low (for quietness) or the fans on high (for best performance) would suit your needs and be alot cheaper.
That's not what i'm asking. I'd like to hear opinions about should I water cool the 7950 or upgrade to 7970 ghz then start cooling. Yes i'm building the whole lope (cpu+gpu). Yes, I'm going overclock.