Well about the dual slot cards, the next ATI cards R520 will also be 2 slot cards so there goes that argument. The current Gen cards compare very well. I personally prefer Nvidia but I have to say that there is not that much difference. Nvidia beats ATI in doom3 and the other way around in HL2. I think one of ATI's biggest problems is their drivers. Nvidia's drivers have been the best for years and I have never had any problems with them. (Duck flames from all over!

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My GFX History:
ATI Mach 64 (ROFL)
ATI RIVA 128 (Was a great card)
Diamond Monster 3D II (3dfx Voodoo 2 8MB)
3dfx Voodoo 3 2000 (Think this was my fav card ever!!! only cost me R700 new)
Gainward Geforce 3
Gainward Geforce 5900 Ultra 256MB
Leadtek Geforce 6800GT
Leadtek I hate cos if you think dual slot cards are big look at this silly card, it is the biggest card I have ever seen! 2.5 slots!!! This silly card is too big to run in SLI on my board!! The only board I know of that can take 2 is the ASUS sli boards.
In the end it all comes down to personal preference, my advice go for the Gainward cards nothing else compares!