it wont really work that way. the Accelero cannot be mounted so the fins are on the backside of the card, the heat will build up on the back of the card, but its nothing the exhaust fan wont be able to handle.
if you go for ducting it, i'd suggest finding a way to duct the fans around the fins, so that the air exits the case in a similar fashion to a dual slot cooler. this wont be difficult, the Accelero S1 cooler comes with a vented PCI filler to place beneath the card. what you need to do is create small panels (or one large one) that will go from the fins to the PCB of the 4850. do this around all possible areas (carefully i might add) so that when you put the card in, it will duct the air passing through towards the back of the case and outward.
you wont need to worry about having upper fans with it like that, however there is an easy way to prevent dead spots of air building up in the top of the case.
http://img150.imageshack.us/my.php?imag ... 010hb7.jpg
^that's a large picture of a P182 build by Ash. what i'd suggest you do, is rather than having the top fans in the case, go all the way to the front of the case with foam. heat wont rise past that foam, the exhaust fan will easily get it out of the case.
heat from the back of the 4850 shouldn't be a problem either as long as there's adequate space between the CPU cooler and the fan. roughly one inch, or 2.5cm should be more than enough. in my own tinkering with a ducted exhaust fan, temperatures actually lowered with more space, as a smaller space just caused turbulence (however fans were of different RPM as well as brand and CFM rating).
you don't want to have too many fans (even quiet ones) in top and side holes of a case like this because there's plenty of room for them (making you want to fill in the spaces) and the fact that even a lot of quiet fans add up in noise. the other issue is that the vent holes are a bit more restricted than they appear on the case pictures.
i'm just poking the less-fan-idea because of how open the front panel is. so much air will pass through that grill when the system is on, even with only 2 or 3 fans in the system (my case has 4 fans total, 2x 120mm, 1x92mm and 1x120mm in the PSU, and if you were to put your hand in front of my case [even an inch away] you can feel the cool air entering the system).
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