AcceleratorX, HD 6770 is not a 6000 series card, based on BART architecture.It is just a relaunched HD 5770 with 3D support through BIOS update and consumes same power as HD 5770. HD 5770 is not at all recommended with a 400W PSU. Check the guru3D recommendation for it or my previous post where I have quoted from there.
And GTS 450 consumes more power than HD 5770/HD 6770. HD 5770 and 6770 have power consumption of 90W to 96W whereas GTS 450 has 121 Watts in load. So a 400W PSU (Gigabyte 460W Superb is actually a 400W PSU, check its official specs) simply not recommended.
A thing about power supply recommendations is that AMD and NVIDIA make them accomodating the vast majority of mediocre power supplies. When NVIDIA or AMD says 500W, they mean an "average" 500W supply. A "good" 400W will still be every bit as good as an "average" 500W supply.
Given the Gigabyte Superb 460W is a rebranded 400W FSP, a fairly good 400W, I'd say it would handle a 5770 just fine (but only barely). I am refraining from recommending 6770s despite being aware they are just 5770s rebranded, because they either come with higher clocks than a 5770 (=more power consumption) or a cheaper custom PCB design that, believe it or not, consumes more power.
The official figures for power consumption are based on NVIDIA and AMD reference designs. Individual manufacturers' non-reference cards will often have different power consumption figures due to the circuitry used.
I have specifically recommended the 3 cards in my earlier post because based on my analysis, these are simply the *best* the OP can get without damaging any component of his/her computer. If there is insufficient power, the performance will be lower, there may be hangups or shutdowns. But since the gap between what the card requires and what the computer offers is not huge, the chance of permanent damage will be very less in the case of those 3 cards.
(Trust me, I've tried......imagine running a GeForce FX 5900 on a 300W supply - it works, even if not recommended. The HD 5770 and GTS 450 consume far less power than that old 5900 and will be safe, if not working great in his/her computer. And if any issues occur, a new PSU can be purchased without risk of damaging the hardware. I have also run a Radeon HD 3870 on a "500W" PSU with less than 40A on the +12V line and that card runs great to this day - 3 years rocking with no electrical instabilities anywhere in the system - checked with multimeter. The 5770 needs less power than the 3870, which is why I'm saying it will probably work).
It's always a good idea to change your PSU every 2-3 years though, because the components age and thus aren't able to store as much energy as they do when they are new. This will cause the effective wattage to drop over the years.
EDIT: Just checked my Radeon HD 5770 box (Sapphire). It says 450W power supply recommended, and they don't mean a top of the line one. Therefore I'm going to put my 2 cents and say that the Gigabyte 460W will definitely suffice for an HD 5770.
