What PSU (make and model) do you have? Make sure it is a good one if you are going with the GTS 450 on a 400w PSU.
The card caries a minimum recommended PSU of 500w. Although reviews show it only consumes (total system) about 275w. However you don't want to be near the limits - I try to stay at about 60% - to have room for issues. To supply 250watts while using only 60% of capacity requires a 458w PSU. Pushing it to 70% still requires a 392w PSU. Also some of the packaged systems have lower quality PSUs, often with lower power to the 12v circuit which powers the motherboard and graphics card.
Note that one review shows system usage of 275w while the other shows 219w. So you can get some idea of the issues in making sure the PSU is strong enough. I try to be conservative in sizing a PSU since after all, powering a PC is not like powering a vacuum cleaner. It is the power, stepped down to tiny amounts, that drives the whole system.
After adding that cautionary note, I think if I already had a GOOD 400w PSU and was upgrading my graphics card, I would definitely consider the GTS 450 for the perfornance benefit.
Yes it is. The GTS450 uses about 88 watts of power in normal 3D gaming, up to 102 watts when being bench marked (furmark). The numbers that rocky had are for total system use (219w-275w) The Antec 400w PSU is fine at that level.