Antec.com - NeoPower 550 SpecSheet
max of 504W on the +12v rails, 504/12= 42 total amperes, only one more than the corsair 550vx. The 550vx is also single rail, unlike the Antec.
AnandTech: The Radeon HD 4850 & 4870: AMD Wins at $199 and $299
Based on the power consumption listed in that article I think the 620hx should be able to run crossfired 4870s.
Corsair 620w SLI Certified Modular ATX Power Supply - CMPSU-620HX - Buy.com
modular but only $120.
Factory overclocked just means they set it higher than the reference design. It's not going to spontaneously fail or anything just because it's set slightly higher.
That PSU is expensive. Get a corsair 750TX for $84 after the $20 MIR.
Corsair TX 750W ATX 12V Power Supply - CMPSU-750TX - Buy.com
edit: yea, it's not modular, but it's much less expensive, and I'd rather spend $100 on faster components than getting a few unnecessary cables out of the way...
I don't know about the 4650, but the 4670 would be a huge improvement.
However, there's a 9800GT for $100 after MIR you might consider:
Newegg.com - MSI N9800GT 512M OC GeForce 9800 GT 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready SLI Supported Video Card - Desktop Graphics / Video Cards
motherboard: Biostar I45
CPU: Q9550
graphics card: 4870X2
RAM: 4GB DDR2
PSU: corsair 750TX
Amounting to ~$1250 or so
The CPU and graphics card I listed are a bit expensive in my opinion, you could drop down to a Q6600 and 1 GB 4870 or something. Just because you have a 2k budget doesn't mean...
Is the $100 premium for the modular one really worth it? Particularly since you say you're on a budget.
So you got unlucky with those other PSUs and lucky with enermax. That's not reason to waste $100 on an unnecessarily expensive PSU.
Get a 750TX for half the price.
Corsair TX 750W ATX 12V Power Supply - CMPSU-750TX - Buy.com
$83 after $20 MIR yet a bit superior to the PSU you have listed. Not modular though.