Tough question. Two single-bit errors in 4GB in an hour and forty minutes is so very minor! Of all the systems I've run Memtest on, I've only seen errors twice. In the first, there were 120 something errors on 2 GB and the PC would randomly crash about every hour. In the second, there were 12,000 something errors on 512MB and the computer only made it a few seconds into booting XP before the BSOD.
You said yourself that your crashes only started happening when you added the two RAM sticks, so do what PCKid said and test them in groups until the system doesn't crash. Maybe try bumping the voltage to 2.15 first.
Hey heres an update, theres definately a faulty stick, but it only seems to happen when the system is hot and under heavy load.
As the system is running 32bit xp it cant see the majority of one stick and will almost certainly never use it, so i have put that stick in the last slot for now and although it crashes occasionally under memory tests its working fine for gaming.
So i think il leave it until i can find warranty info.
The memory that xp can't see is being used for something vga card related. Just take the bad stick out and run 3gb. You won't get dual channel, but 3gb is still better, and will run almost as fast. RMA the bad stick.