there is absolutely nothing wrong to put a 430 GT into your motherboard.
there are absolutely no incompatibilities.

as long as it is a PCI-E graphic card, it should be ok.

i can see you are targeting for economic graphic cards in Nvidia.

but i don't recommend to take the 8400 GS.
i know its cheap, but it is more than 2 years out of date,
you can get even better and up to date graphic cards,

unless you get it second hand and for FREE, then there is a price advantage.
otherwise, get a 430 GT.

it is almost as cheap, and the price difference should be less than significant,
compared to the performance difference.

if you really are desperate for cheaper cards,
get a N210 kind. it is definitely better than 8400GS.
but it still wont be satisfying to play games on it.

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