I finally set-up my PC to do some memory testing with Memtest86.

I started testing some of the memory I got laying around, and most of it passed all the tests.

I got 2 sticks of 512mb that have errors on some of the tests.

Yet, when I put them in a system, it boots up and seems to run OK.

What does this mean?

Are these sticks really good, or does it mean the system my crash using them?

i would think that it means if ur system is at full load with the memory it might become unstable 

Yep it just means your system is unstable. I bet you can't run prime95 for 24 hours with those sticks of memory and not have a failure. You may be able to boot with bad memory and not notice anything wrong right away but something is bound to crash or give errors at some point.

Pentium691:

I finally set-up my PC to do some memory testing with Memtest86.

I started testing some of the memory I got laying around, and most of it passed all the tests.

I got 2 sticks of 512mb that have errors on some of the tests.

Yet, when I put them in a system, it boots up and seems to run OK.

What does this mean?

Are these sticks really good, or does it mean the system my crash using them?

At least when you get a BSOD or shut-down (crash) you will know what caused the problem! Try underclocking your RAM and see if the errors disappear. MemTest is great, but you might also want employ Super Pi to check your system's stability.