Well, based on this..
a system equipped with 7770 just consume less than 250 watt while gaming..
so, your vx450 actually more than enough..
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let's look your ms pro-550. well, a generic psu.
the label says. +3.3V @28A, +5V@34A, +12V@16A, -12V@1A, -5V@0.5A, +5VSB@2A
this is an ancient PSU design!!
It doesn't list total power available on the 12V rail so you have to assume that it can deliver 16A x 12V = 192W. So that means that your "550W" PSU can only deliver ~450W.
90% of the power draw on modern PCs is on the 12V rail by the CPU and GPU so your PSU is realistically about a 200W PSU.
That's mean s, a "550W" PSU actually delivering about 200W of USABLE power.
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well, conclusion is, your vx450 is thousand times better than your "new" psu..
then yes, you've already throw your money away..