I'm not reading all the arguing back and forth, but here's my viewpoint that you can do with as you please:
The argument at this point should not be is SM3.0 the way to go or not. We have seen that developers are developing for SM3.0. Eventually SM3.0 will be in use as SM2.0 is in fairly extensive use now.
I think the argument should be something more along the lines of:
When games are actually using significant SM3.0 will any of the hardware we have now matter?
I'm thinking back to SM2.0 and DX9. nVidia had the FX series that performed fine on initial DX9 titles that had some, but not extensive, use of DX9 features. Games progressively added more DX9, and the FX series ultimately showed to have inadequate at DX9 performance as we've all seen in the current generation games.
Are they doing the same thing with SM3.0?
I don't know. But from what I'm seeing, I think it may be so. I think nVidia likes to have a long list of features they can advertise, and SM3.0 may be a fairly superficial implementation in the 6xxx series, much as DX9 was a fairly superficial implementation in the 5xxx series (especially the 5200, that card was a total joke). Note my use of the word 'may'. I am speculating and not accusing.
The argument only exists because nVidia cards are going for a significant premium over ATi cards that lack the feature. If the 6800GTs were to come down in price to where the x800XL is, the point would be moot, because everyone would be buying 6800GTs. They arent, and we can only speculate as to why the price is staying high. Is it because nVidia sees SM3.0 as a premium feture worth a premium price? Possibly. It could also be that their cards simply cost more to produce and that ATi really does get a significant price advantage from the smaller die of the x800XL.
My feeling is that there is enough doubt involved that the CURRENT SM3.0 cards available are not worth considering exclusively because of their SM3.0 support. Considering, yes. For performance and value, but exclusively because of SM3.0 support, I don't think so. I can't trust nVidia that much after getting burned on the 5xxx series. If you're buying PCI-e you pay a VERY steep premium for SM3.0 support given the performance of the x800XL. I'm willing to bet that when the time comes when SM3.0 is REALLY in primetime, you'll be able to sell off an x800XL for a reasonable price, and the difference in used prices between an x800XL and 6800GT will be less than the current difference between an x800XL and 6800GT is today, so you would come out ahead anyway.
AGP is another animal. The 6800 is cheaper for AGP and the x800XL is more expensive. The gap is narrower and the decision is more of a toss up.
I am considering upgrading to a Venice/939, and if I do I will be going PCI-e. To me the PCI-e card that makes the most sense to me is the x800XL. If I were on a significant budget I'd get the 6600 (non-GT). I don't favor brand, only value.