if you forgot the GA in the name (GIGABYTE GA-EP35C-DS3R) then yes, it should fit, your board has an x16 slot, and the 4850 might be pci-e 2.0, its backwards compatible with the pci-e x16 slots.
the 4850 seems like its the best bang for the buck for now.
heres some benches
i dont know the AUS currency compared to the US currency though.
EDIT, i found out the currency, seems like its about the same as US
heres a 4870 for about $274 US currency, which this card is better than the 4850, but dont know by how much
i would just get the 4850 for now to save money since the 4850 is almost $100 cheaper.