I bet you've already gotten the card, and you've made a good decision no matter which you choose.
If you plan on getting one down the road, get the 6600GT. It can play high settings on games such as HL2/CS:Source, high/medium on BF2, and medium on FEAR (without soft shadows)
The 6800GS can play the above pretty much all on high
Both are great cards and will tide you over until the g80 series.