1. The GTS450 is a couple of tiers higher than the HD6670, but the chart does not indicate it's a GDDR5 version. If it is, then having DDR3 instead may bring it down to about the same level.
2. At your resolution, the difference in amount memory will make no difference.
3. The HD6670 uses less power. If your PSU is not a good one (e.g. Seasonic, Antec, or Corsair), it may not be enough for the GTS450. What brand and model is it?