RAM OC'ing noob needs help

Hey, I'm trying to get 4x1 GB of RAM to run stable on my system. 2x1 GB works fine, but plugging in the other two sticks gives me a myriad of errors, from the "header checksum does not match the computer checksum", to not posting, etc. I've toyed with the timings, voltages (RAM and NB), and other stuff in the BIOS but I can only get it to boot into Windows randomly with 4x1 GB.

I'm running Vista Ultimate 64, mobo is a P5K-E with the newest BIOS, RAM is G.Skill DDR2 PC2-6400 (4-4-4-12, 2.0-2.1V). I've tried the various JEDEC and EPP settings from CPU-Z and toyed with them slightly, but to no avail, so I'm not sure what to do next. And yes, I have googled the fcuk out of it.

Any ideas?

First thing to do is run memtest86+ on all four sticks, two at a time as four won't work.
If/when it fails, try just one of those two sticks, if that works try the other one. If they both work, something funny is going on. If one fails, you have a stick of bad ram.

After that, try giving the northbridge a voltage bump, sometimes the NB needs some help when it's driving four sticks of ram.

yeah the northbridge controls ram so upping the voltages is almost always the solution

Ok, did some testing (not with memtest86+...) and it turns out that my computer boots into windows with every other RAM stick except one. If this one is plugged in along with any of the others, the problems described above occur.

So I want one more gig of RAM. I'm assuming I should go for exactly the same brand and spec as the ones I'm using right now. How important is this? Can I go for another brand's DDR2-800 PC2-6400 with the same timings and expect stability?

mixing types of ram and timings is not good. it will work but there will be a myriad of problems and a general lack of stability