If you wanna run a Radeon in "SLI" mode you're gonna have to get a Alienware AXL system.
My plan is similiar to others with regards to SLI, except I'm gonna wait for a price drop on the GTs towards the end of next year, as I doubt I'll have 300 quid to blow on another card until then anyway.
Still, if the card drops to say .. 60% of its original price by then, like previous geneations have, thats a pretty good boost you'll get for the price. Maybe economically better than buying a next gen card when they come out.
