A 9600 GT would give you better performance and you've got the extra power input for it.
A 9800 GT or a 4830 should work as well , they use about the same power as the old 7900 GTO that I ran in my D8400.
Performance with either of those would be better but won't match what you see others getting due to the P4 in your machine.
I'd say get a Radeon 4670. Unless the resolution on your monitor is higher than 1440x900 you won't really notice too much of a difference going with a more powerful card simply because your CPU will be holding you back. The Radeon 4670 is much faster than the 9400GT. If you feel the 4670 is just a we bit too much then consider at least getting a 4650 which will still be a pretty powerful card for your system.