Ok, let's see
. HD4830 - TDP=110W, 2xHD4830=220W, GTX280 - TDP=235. So we have 15W of difference. I don't think that it's important. Let's look on another things. We need CrossFire capable MB, but in case of single card we can use any MB with PCI-E. When you'll OC two HD4830 and GTX280, TDP of two ATI cards can be bigger, than one GTX280. I will not discuss advantages of nVidia GPUs, because it's another matter. Let's just say that it's personally choise of each user what system he wants to build.
What I really want to say? In case of BFG 9800GT with passive cooling, SLI capability is really not very important thing. You'll need a storm in your case to cool your passive SLI and, I suppose, passive CPU (in other case such card is just unreasonable purchase
) with chipset in addition.
If you want to have silence, you must lose some performance.