Actually, DX10 runs like shit, relatively speaking. They're still working on native DX10 games, every game so far was just a port and the difference in DX version was just not there. DX10 capable cards run DX9 just fine.
Even though it's an older card, the 1950 pro is surprisingly powerful, Newegg link. I would recommend you go and find a used one, they should be far less pricey.
For a little better investment, the 3850 would be good. Newegg clicky
For both cards the 512MB version would be faster with high AA and texture quality, but the difference is smaller than you would expect.
Both the 7300GT and 8400GS are the lowest of the low-end cards. You may be able to run at 800*600 lowest quality with both, but that's it.