Once again the 550ti consumes double the power of the 7750 and really there is not need to go Nvidia right now Radeon just has better options at every price point.
you think just 400? I mean i might as well grab some more wattage with about a $150 g card
With the card I recommended in my above post you should be alright with your 400watt PSU what I am suggesting is use it try it out and if it works you saved money if you have problems then the purchase of a new PSU will be justified but I think you will be alright.
This is the crappy and OLD PSU I am using problem free to power a GTX 275 which takes almost double the power to run over the 6850 I recommend you get.