Most 64-bit cards should work in 32-bit slots. Since the PCI standard allows for a optional 64-bit bus, I'd assume you should only suffer a performance hit, nothing that should impair video playback of most media. How much of a performance hit, I wouldn't know. Depends on how taxing the particular card is on the 32-bit bus width of the PCI slot.
As unreliable as Wikipedia is, they do have a nice article on the PCI slot: http://en.wikipedia.org/wiki/PCI-X#Mixing_of_32-bit_and_64-bit_PCI_cards_in_different_width_slots Even includes a short write-up on mixing 32-bit and 64-bit cards and slots:
I'd personally go with the GT430. It can at least let you do some light gaming/casual gaming while the GT520 performs worse overall in gaming (GDDR3 vs. GDDR5 as well as number of SP, stream processor, differences and clock differences).