You've measured 450W at the wall using a Watt-Meter? That's nice, so you know exactly where you stand.
PSUs are pretty efficient these days, and their efficiency curves are much flatter across the duty cycle than days of yore. We're talking >87% at 20% load, >90% at 50% load, and >87% at 100% load.
For that reason, I don't think 750W (while more than you need) is bad in any way necessarily.
Sounds like you should be looking for a PSU with semi-passive fan operation. My favorite two are:
EVGA G3 - Their 650W unit
is on sale for $77 after MIR. Advertised as fanless <20% load and reviews
seem to support that.
Seasonic Focus PLUS
(make sure it's the plus model) - 650W for $70 after MIR
Advertised as fanless <30% load, but according to reviews
that might go closer to 50% load...YMMV.
Looks like the 750W units from both manufacturers are sitting around $100. Also the Seasonic Prime Ultra 750W
(with 12 year warranty) is discounted to $100 right now (until Monday). And the Seasonic Focus+ Platinum 750W
is $105 after MIR.
From the reviews I've read, there's rarely much of a difference between 80+ Gold and 80+ Platinum PSUs in the same family, especially when they have similar cost and warranty. Most of the time, the gold units pass gold ratings with flying colors and the platinum units barely
squeak by (keep in mind, we're talking 2% efficiency difference between gold & platinum). For that reason, I wouldn't put much weight into a Platinum-over-Gold choice. You really have to step up to the Titanium models to get a noticeable difference in platform quality/efficiency, but those PSUs get pretty spendy.
Main: i5-3570K, ASRock Z77 Pro4-M, MSI RX480 8G, 500GB Crucial BX100, 2 TB Samsung EcoGreen F4, 16GB 1600MHz G.Skill @1.25V, EVGA 550-G2, Silverstone PS07B
HTPC: A8-5600K, MSI FM2-A75IA-E53, 4TB Seagate SSHD, 8GB 1866MHz G.Skill, Crosley D-25 Case Mod