To Battlefield 3, no. Trust me, I tried it for fun and man was it unplayable even at lowest. (8400GS).
8400GS isn't designed for gaming at all. It's a good card for some of the 2000-05 games, but not for the current 2010-11 games.
For MW3, not so sure since no benchmarks have come about. Also, I don't think it is really demanding at all, since the game is using the same engine as COD4 with some modifications to it.
probably, hard to say, it's a down clocked 5670 with slower memory module.
well according to what I can find it's slower than 4830, which is below the minimum spec, but since you have them in SLI it might be able to run low.