THG's review seems to support yes for it maintaing over 60 FPS, however I tend to doubt said review given that my 6800GT won't maintain whatever ~70FPS they claim it capable of...it doesn't seem even close (then again, I am not using settings remotely near theirs (I have AF and AA turned on, not very high but it's still on))
Given the power of 7800GTX on it's own, a second one would probably allow what your wanting but maybe not at the resolution your wanting...(you didn't state which reso your looking at, given the SLI motherboard and 7800GTX, i'm guessing your gaming on a fairly high dollar system, so probably 1280x1024 or higher)
The question i'd have is what other options do you have? You could get a single 7800GTX 512, which if put in SLI would disable half it's memory, so that's a bit of a waste of 256MB
You could get an X1800XT, which is already reported to have odd performance in some games, AOE3 being one of them (however you didn't state AOE3 as a played game, so that might not be an issue for you)
You could get an X1900XT, which will probably be equivalent to an X1800XT on steroids in terms of performance...
while a second 7800GTX would probably cost the least, bench the highest, and run games with AA and AF the highest due to the extra processing power...
I'd suggest the 7800GTX based purely on the other options in the same performance range