Trotter is the Man
While the Corsair 400w is a great power supply I'm sure your new system is straining it harder than it ever has. I would go ahead and upgrade it before the hard drive.
This Corsair is only $75 after a rebate.
Newegg.com - CORSAIR Enthusiast Series CMPSU-650TX 650W ATX12V / EPS12V SLI Ready CrossFire Ready 80 PLUS Certified Active PFC Compatible with Core i7 Power Supply
This Antec is only $70 after a rebate
Newegg.com - Antec High Current Gamer Series HCG-620 620W ATX12V v2.3 / EPS12V v2.91 80 PLUS BRONZE Certified Active PFC Power Supply
This OCZ is only $50 after a rebate, but it does look like a few of them have died early. But OCZ does have a pretty good support team.
Newegg.com - OCZ ModXStream Pro 700W Modular High Performance Power Supply compatible with Intel Sandybridge Core i3 i5 i7 and AMD Phenom
As far as the Win7 rating, it's really a generic rating and I wouldn't let it influence my decision making. With that said, if you want a big boost in your Windows load times you'll want a SSD, Solid State Drive. They help your load times with games and apps also, but it's not as noticeable. They are fairly expensive so you would want to buy a drive big enough for Windows and your main applications and a few games. Then you use your old drive for data, temp files, page file, etc... OCZ is pretty much the biggest player in SSD. They generally have the best price to performance but you never know what might be on sale at the time.
This is a 1st generation drive but it's really only big enough for your OS and the bare minimum of apps. I have this exact same drive with WinXPx64, Autocad, Photoshop, MS Office and a few other smaller programs and it's almost full. But it's priced right for a starter drive.
Newegg.com - OCZ Vertex Series OCZSSD2-1VTX30GXXX 2.5" 30GB SATA II MLC Internal Solid State Drive (SSD)
This 60GB drive is $95, I also have one of these, I'm running Win7 on it as I type. It hold Win7 all my programs and 5 or 6 games, this would be my recommendation.
Newegg.com - OCZ Agility 2 OCZSSD2-2AGTE60G 2.5" 60GB SATA II MLC Internal Solid State Drive (SSD)