Originally Posted by SiM 
Fair enough... I didn't know that... so it can't use both sets of vram at the same time?
Correct. Making both the 3870X2 and the 9800 GX2 a pikey bodge. If they were designed properly, then they would have either designed the Chips in the first place with a Hypertransport-esque interconnect running much faster than any Hypertransport currently available* or modified ATi's 512 bit ringbus to run off die but in the same module effectively sharing the memory controllers on each die.
Imagine that - Crossfire on a single GPU, except without crossfire and with near linear performance scaling!
The problem with doing it properly is they would need a different core for the X2/GX2 cards or have lots of useless silicon on every single GPU card bumping up the cost of single GPU cards.
Before anyone lays into me, if the cards are designed right, the copper paths between the GPU's could be short and noise free enough to easily run as fast as the memory interfaces themselves. There also wouldn't need to be much of a latency hit for accessing memory connected to the other GPU.
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