The new Nvidia graphics cards come under the band GF100 not G300.
Allot of your questions are still a matter of debate atm, both Fermi and 5000 series are meant to offer triple screen support however the 5000 series can do it with a single card and display port compatible monitors.
Currently the 5870 and above are a fair bit faster then the GTX295 however the demonstration version of the first Fermi card has potentiality double the performance of the GTX295 which would make is fly past everything ATI offer atm including there duel GPU card, but I should mention thats just on paper atm.
There are also potentiality size and heat issues with the Fermi card but again as the press haven't had there hands on one yet thats speculation.
Personally if your not sure if you want an Nvidia card or an ATI one I would wait a while until DX11 is more widely used and till Fermi is out and has a few driver versions behind it.
I'm currently running 2 GTX295s in standard mode not SLI, using TH2Go at 5040x1050@59 and I've not come across a game other then Crysis and GTAIV that I can't run at max detail x4 AA.
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