|your motherboard has a PCI express x16 slot, which is where you stick the graphics card. This is the newest slot standard, so any new graphics card will fit in this slot.|
you need to make sure, though, that your Power Supply Unit (psu) will support the new card. The 9400 is not very power hungry, so your PSU should be fine, but you want to double check that sort of thing just to make sure.
btw, is this for gaming? if you want a little muscle for gaming, consider a slightly more significant upgrade: both the 8400 and 9400 models were low-end, compared to cards like the 8800 and 9800. There is also a newer series from nvidia, 2xx (like 260, 280, 295, etc). If you're not locked into NVIDIA, you have even more options in ATI cards as well.
Post PSU specs and a price range, and people here can help you pick a good hardware replacement.