If I remember all the 6800GT+U reviews correctly, I think a GT will run almost as well as a U in 12x10 4x4, and it has the added benefit of being a one-slot solution. The U, though, has the advantage of dual DVI outputs (in addition to a 20% faster core and 10% faster memory), which may be important if you want to drive two 3D-accelerated LCDs off of the U (otherwise, just buy a cheapo second PCI card).
If the U's higher price, extra case space, and potentially more demanding power requirements don't phase you, though, then go for it. If the price is an issue, I don't think you'll feel too bad with a GT, and that $150 can buy you Doom 3, Half-Life 2, and a third shader-showcase game of your choice.
Here's a review that compares a GT and U with a P4C 2.8GHz and a much faster A64 3800+. I don't think they used AA+AF on all tests, though.
This Firingsquad Far Cry article may be more useful, as it shows the U and GT comparing in many levels of a shader-heavy title with AA+AF at 12x10. This may be more indicative of the performance you can expect out of D3, HL2, and other GPU-intensive titles. The differences do range between the core and memory clock differences, 10-15%. The question is whether you think going from 52 to 60fps is worth an extra $150. You may stretch that difference even more, though, as the U should be able to OC better due to its better HSF.
If I were you, though, I'd take the GT and enjoy it now, and if you're worried about losing out on some performance, stash that $150 away to buy a 6900 when it comes out this fall.
If the U's higher price, extra case space, and potentially more demanding power requirements don't phase you, though, then go for it. If the price is an issue, I don't think you'll feel too bad with a GT, and that $150 can buy you Doom 3, Half-Life 2, and a third shader-showcase game of your choice.
Here's a review that compares a GT and U with a P4C 2.8GHz and a much faster A64 3800+. I don't think they used AA+AF on all tests, though.
This Firingsquad Far Cry article may be more useful, as it shows the U and GT comparing in many levels of a shader-heavy title with AA+AF at 12x10. This may be more indicative of the performance you can expect out of D3, HL2, and other GPU-intensive titles. The differences do range between the core and memory clock differences, 10-15%. The question is whether you think going from 52 to 60fps is worth an extra $150. You may stretch that difference even more, though, as the U should be able to OC better due to its better HSF.
If I were you, though, I'd take the GT and enjoy it now, and if you're worried about losing out on some performance, stash that $150 away to buy a 6900 when it comes out this fall.