How often does RAM fail? I had two matched pairs of
512MB DDR2 in my system, then today, apps started
randomly crashing. So I go to reboot, and it kernel panics
on boot.

So I remove the new stick of RAM (same as what the
computer came with, exact model Samsung DDR2), and it
boots fine.. So I try booting with only the new stick, and
it crashes..

What I have here, is a nice stick of Samsung DDR2, that
has already failed, while the other stick that came with the
IBM (same make and model RAM), is still working..

The new stick "was" working for 4 days..

What in the world happened?

Where did you get the "new" stick from?

You had an elctronic componet go bad, is there a waranty with memory stick?

Did you unplug the computer prior to installing the RAM? How about static precautions?

One other cause could be the required voltage. Some RAM runs at slightly different voltage. If this happens to be the case it could have an impact. That is probably not the case. Could be as sited, just a failed stick. Most RAM comes with a lifetime warranty.

Run memtest86.

Got the RAM on eBay.

It's the right RAM, as I made a point, it's the SAME stick as
what came with the computer, which was stock.

I did ALL the precessions, as I'm a technician in the field
myself.

I could have bet everything I owned that the RAM's source was fleabay, but decided not to pre-judge the source ... LOL!

In all fairness, it is possible to buy good memory from eBay, because I have sold both brand new & pre-owned memory on there myself. But I have yet to buy memory from there. Who knows ... you may have been sold "dodgy" RAM as many folks like to refer to faulty memory on the internet.

If the two memory modules are identical as you've stated & one works while the other doesn't, it is hard not to draw a quick conclusion as to the status of the non-working stick.

Good luck!

Thing is, the stick was working for like 5 days, until last
night, I come to a kernel paniced OS X...

"until last night, I come to a kernel paniced OS X..."

You've mentioned before that you installed the Mac OS on an IBM machine. Now, our dog probably knows more about OS X than I do, but I know for a fact that 10.5 is better off on a Mac, not an IBM. Could it be that a less-than-optimal driver is causing the error? Does your P4 support SSE3?

The creme de la creme of Socket 939:
Opty 185 @ 3.2GHz
SLI'ed GTS-640s, both flashed to 625/1458/1950
4GB PC3200
Blu-Ray/HD-DVD, X-Fi
A8N32-SLI Deluxe
3DMark06: 13896

It runs fine on the IBM, because the chipset is the same as
a real Mac, and yes, it has SSE3, has all proper instruction
sets..

I'm running fine now, on the stock stick of RAM.

The kernel panic is showing bad memory references, and
memtest has failed. So the stick is bad it seems..

(And I know for a fact, that if an IBM and Mac contain the
same chipset, minus the CPU, it will run fine, as long as
the CPU has the instruction set, and EFI is loaded at boot
time, which it is.)

Oh well well well.. I think I found the problem..

A tiny little bug, what you call them, the ones that look like
moving dots, was bridged between two test points.. So far
the IBM has booted up with the ebay stick now... Will do a
memtest tomorrow morning on this..