I've been using two 256mb samsung pc3200 sticks lately, and today I got one 512mb pc3200 PNY stick, and it doesn't work together with the other two. It's stable until right before the windows boot-screen, and then it restarts. If I only have one samsung memory together with the new one, it doesn't get past the POST screen. And if i'm just using the new one, it works.  
 
I guess this is quite common but is there some way to fix this?
 
I have an abit nf7-s motherboard.

Some chipsets are very sensitive to mixed memory configurations. The only solution is to change some of the memory. While samsung makes some of the best memory circuits PNY probably uses some poor curcuits. And since your motherboard has a nForce2-chip it is more sensitive to poor curcuits. Memorysticks from the same manufacturer are usually the best. But since your chipset is more sensitive to poor memory curcuits it could be a problem to run two PNY-sticks at the same time. The best configuration for you is 2x512MB since your motherboard can run the memory in dualchannel mode.

Might try setting the memory timings loose.  Could be that the memory in bank 1 is faster than the PNY and the mobo is reading the SPD of the first bank which could cause system to hang if the second bank can not meet that setting.

System should default to the timings of the slowest DIMM.

I thought (on Intel rigs) that it defaulted to the first banks SPD setting, so if the second bank was slower, it would need to be manually set.

Regardless, worth checking and loosening timings, since common for mobo to not read SPD preset memory timing correctly.

I don't know if it will make a difference or not here is what Abit says:
 
NOTE: We suggest you to install DDR SDRAM modules from DIMM3 to DIMM1 sockets in order.
 
I personally would start withe the 512MB in DIMM3

DIMM3 would be the one that's on the right, right? The one that's a bit seperate (dual channel). I've had the samsung memory in the first two and the pny in the last, seems most logical. I've tried moving them around too..
 
Anyhow, I tried with loose setting, even with the most loose that my motherboard could 3-7-7-15, but it didn't work, it restarts. Sometimes when I changed the latency, right after I pressed yes on "save and exit setup", the computer shut itself off and started sounding like an alarm. And then when I powered it on again it continued as if nothing had happened, saving to cmos...
 
Anyhow, my friend has some samsung meories in his computer and if the pny memory works in his we can trade them. But do you think it will? He's got ic7-g or something and I think he has 3x256 samsung memory, but not all of the same model.

Your at the point of what you got ain't working, so try the swap and see.