It is a very close race. The 7900GT is generally cheaper and has far better overclocking potential. As far as I've seen, the 7800GTX 512 still outperforms it in many instances at stock settings, but the 7900GT outdoes it easily when mildly OCed.
If you have either one, upgrade isnt really an necessity. If you have neither, get the one on the left. the 79 are 90nm so run cooler, a little faster than its counter part, and OC like crazy. Oh and 90 are cheaper.