You could certainly get away with a HD6450, although you'd have to reduce the in game settings considerably.
If you went with the ASUS HD6670 and your PSU could power the card properly then your PC may not start or may crash when playing games, the PSU could overheat or your PC may just restart under load.
Mdent will be fine with an HD 6670 or 7750. Dell uses good PSUs in their Inspirons. I ran an overclocked HD 5670 in mine for a year, and my PSU was always silent - no heat problems at all. Like you said, even under load, the system will barely draw 125w.