Both are top notch PSU IMO yes the seasonic is overkill. And yes corsair is built on a seasonic platform so the guts are mainly the same sounds like the seasonic has a few extra features. I personally would just get the Corsair.
Corsair and Seasonic are two brands that have a reputation for high quality power supplies that consistently earn high marks in technical reviews. They are reliable, stable, and come with a 5 year warranty. Some of the newer models come with a 7 year warranty.
I have the new Seasonic X650 gold certified power supply in my new Intel Core i7 860 system.
I also have a Corsair HX620 in my emergency back-up pc. The Corsair HX620 was my primary power supply until I purchased the Seasonic X-650 a few months ago.
If it is within your budget, then I recommend the Seasonic X-650.
The Corsair HX650 is made by Seasonic.
I can confirm that the Seasonic X-650 is a very quiet psu due to the hybrid cooling. Voltage output is very stable even during torture tests.
When we use the term "overkill" we usually mean a high wattage power supply that delivers a lot more power than a system actually requires. For example, one might have a pc system with just one energy efficient video card. The system might only require a total of 300 watts while playing games. Installing a 1,000 watt power supply when only 300 watts are required would be overkill.
If you provide a list of pc components we could advise you whether a 650 watt power supply is overkill or not.
The Seasonic X-650 Gold power supply is more than enough to power your system. In fact it can power your system with two ATI Radeon HD 5830 video cards operating in Crossfire mode. You'll be ready for future upgrades.
The Olympia is no longer for sale and the SG-650 has been outta stock on newegg for about 2 months now.....my guess is they are waiting for the new 650 to arrive. It's still available on some sites. Jonnyguru hasn't tested the HX-650 but the HX-850 did garner a 10.0 performance rating also. Of course that's no guarantee that jonny would give the 650 the same rating but in general the HX series has been rated quite high on most sites.
Between the HX and X-650, I'd grab the Seasonic for its quietness. The full modular thing I think is really a non issue. Personally, I prefer hybrid modular as there's really no point in adding the extra cost and resistance (small as it may be) for an extra connector for those cables (24 pin, 8 pin EPS, 1st PCI-E, 1st SATA) that you must use.