yup, shim is higher so other sinks won't work without help. an extra thick layer of thermal tape doesn't sound ideal but might work, filing down a little bit of the base of heatsink so sticking out bit fits inside the shim. possibly removing the shim though it sounds epoxied on, i believe the 8800 series ones are just screwed down and easy to remove.
you'll have to see, you can always remove the shroud of the stock heatsink and take the fan off, rig up a quiet fan to it. i think i might be using something along the lines of a 120mm fan with 120-80mm adaptor and have it roughly blowing down onto the sink and see how that works.
i've said it before, and again, and will keep saying it. blower fans are pure crap, tiny airflow, higher cfm blowers = insane amount of noise, getting heat out of case is pointless, at the moment my case is almost silent but has 3x 120mm's and a 200mm at the top(antec 900), i don't care if heat from card is inside case as that heat is removed very very quickly. still 90-95% of users have few or no pci cards, no crossfire or sli, so 95% of us could have a gfx card with a thermalright ultra extreme sized heatsink underneath it, insanely good silent cooling and most of us could fit that in fine. theres no reason for the stock sink to be small, or have crappy airflow and high speed fans.
why does the pitifully small zalman heatsink work better than a x1900xtx stock sink, because its got a real fan on it, despite probably 1/5 of the copper and like 1/20th of the surface area.
EDIT:_, theres also no reason the pci slots have to be so restricted, if you had pci slots with much thinner metal so much bigger holes basically, then you could have a big heatsink filling the space under the card, and have the fan blow most of the hot air out the pci slots if you want. i get that some people do crossfire/sli, and some people have sound cards(way fewer than you think), so do some cards with smaller cooling solutions, but theres probably only 5 or 10 out of 100 people on these forums that can't fit a massive sink under there.
And going for some third party HSF, looking at the 2900 card it looks like the required holes are the same as on the Radeon 1900,
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