Actually the min requirement is GeForce 7900, so the 8400 shouldn't be a problem.
The rest of your system should also be fine.
Actually, if you do some research on Nvidia graphics cards, you'll find that the 7900 is a far better card than the 8400. Nvidia name their cards so that the first number indicates the generation (meaning that the 8400 is newer) and the second number indicates its range (eg 8600 is mid-range, 8800 is top-end, wheras the 8500, 8400, or lower are not designed for gaming). This confuses many people who are new to building PCs and means that companies can get away with selling crappy graphics cards to people without them knowing - hence all the people complaining about crap performance on their PCs.
Did you read this?
oh.15-20.