I'd say 720p is passable for early DX9, but the NVS 300 can perform at this level with an overclock that puts it in the 20 W TDP ballpark. This is the minimalist mode basically. Then the question is if you want more do you want to max out the settings or do you want medium.

NVS 300/GeForce 210 is just too bandwidth limited when GT 320 provides 3.5x more FPS for 2x more power.

The NVS 300 and the GT 210 aren't the same card, the GT 210 is slower and has a higher TDP. If you already have a GT 320 then you may find a use for it but it's not a particularly low TDP model and for 30% more TDP you get the HD 7750 which is 3x faster. So this would be the graveyard zone.

NVS 300 average FPS in various 3DMarks on high, 1024 x 768 with vsync at 60 on a Haswell in Windows 7:

199 61.5
22000 59.9
32001 59.9
42003 60

So comfortable in DX6-8/9. Total system draw 35 W, so make a cheap low profile low power retro machine.

The next step up should be something categorical, like 1600 x 1200, 1080p in later DX9, or high SSAA. 1600 x 1200 is probably doable with the NVS 300, 1080p in later DX9 you want something in the HD 7750 range or better, and for SSAA Radeons are more convenient.