The 3700+ / 7800GT is the safest, best balanced option for gaming. It will have a longer useful life, as the 7800GT is on average about 33% faster than the 6800 GS in games.
This 33 % translates to about a year of added longevity
I have seen a test (X-Bit Lab I think) showing that a 2.2 Ghz A64 processor, like the 3700+ is about the best match for the 7800 GT. You don't want your GPU to be limited by the processor, and you don't want to spend additionnal money on the processor if there is very little performance increase.
With the 3800+ X2 / 6800GS combo, you will be trying to rely more on a stronger processor to keep up-to-date in games. But better results in games with the X2 processors is dependent on software patches and hope that future games will better take advantage of dual-core, probably with mixed results. And in any case, the most demanding games are limited by the GPU, not the processor.
Also the 6800GS is based on the previous generation of nVidia GPU's. There are more functionnalities in the 7 series.
I know everybody is saying dual-core is the future, but I think your safest bet at the current time for a gaming system that will stay in the race for the longest time is to go with the faster GPU (7800GT) with a well-matched single-core CPU (3700+).