I've got a perfectly good, but 7 years old (though it hasn't been used much in the last 4 years) SeaSonic S12 500W and was wondering if I could use it in my next new PC build with Intel Core i5?

The S12 is a good PSU, I would use it. With a more modern PSU you would get better efficiency and the system would draw a bit less power, but not enough to justify getting a new unit imo.

New units, like Super Flower Golden King 550W or an identical model from Kingwin are superior as they are semi-passive: in normal loads (<400W) they are 100% silent: the fan doesn't rotate at all.

As it's an older ATX design, it's got split 12V rails that total to ~400W. So, good for up to any single gaming card build.

Upside to re-use:
- no $'s spent
- one less item getting recycled

Downside:
- lower efficiency at idle and load
- not as quiet as newer high efficiency designs

Thanks. That's kinda what I was thinking too. Adequate, but less efficient than a modern unit.

Aging might worry me if it were to use for GTX580 SLI. Otherwise, it should be fine for light use like single 6670.
Btw, my S12-430 is still running strong in my dad's PC.

Thanks. Yeah... no... I had to Google both of those gfx cards because I didn't know what they were.
So no, I'm not planning on using anything like that. In fact, I'm holding out for Ivy Bridge because I'd like to use the integrated HD 4000 graphics in that CPU.
Ivy Bridge is also supposed to use less power than Sandy Bridge. Though don't know whether any potential energy savings would be lost by using the S12 anyway.

Efficiency loss - sorta depends on what you compare it to...an 80+ Gold or Platinum ATX PSU or a 120W Pico-PSU based solution...plus I don't know how far efficiency degrades on your S12 as you go below 65W DC. Might be in the 60%'s.

Comparing to the recently reviewed SS-T350TGM:
- If your system idles at ~40W DC, and assuming 65% efficiency for the S12, that's ~8W AC loss.
- Watching an H.264 movie might be 50W DC, and call it ~7.5W loss over the SS-350TGM.
- Heavy load of 100W leads to ~6% efficiency difference or 6W loss.
Worst case, call it 8W all the time. In my neck of the woods, that's ~$8/yr if you leave it on and idling all year.

For comparison purpose:
I was a bit surprised to learn that efficiency drop is so large.
Noise-wise, S12 is ok. Efficiency-wise, it's no no. It's the least efficient PSU in the comparison.

System spec:
AMD 4850e 1G @ 0.85V
Gigabyte MA74GM-S2 rev 4.3
DDR2-667 2GBx2
Hitachi 3’5 500GB 7200RPM x1
80mm Fans x2 @ 5v
DVD burner x1

Idle power consumption, measured from the wall using Powerangel