There is never a guarantee that RAM will work. RAM is the trickiest and most finicky component when it comes to actually working and is best if all modules are from the a single kit. if you have 4 modules that are from two separate kits that are identical it may still not work although there doesn't seem to be any reason why it doesn't.
It might work, it might not. If you have a dual channel motherboard (any board with two or four memory slots is dual channel except for the very new SB-E boards) then yes you may see a performance decrease. if you have a tri channel board (you probably have a dual channel, it is more common. Tri channel has 3 or 6 memory slots) then you would see a slight performance increase.
The 800 is more a speed thing than a voltage thing.
Regardless, the ideal is to have the exact same part number rather than different part numbers. This makes maximally sure that there are no catastrophic differences in the RAM.
Having one set be DDR2 and the other set be DDR3 would be one of those catastrophic differences.
When I looked up KHX6400D2K2/2G it looked like it was 2 x 1GB. If so, you won't get those 2x 1GBs into 1 RAM slot.
Also, the part where it says KHX.....6400....whatever means it is PC2-6400 RAM. The part where your current ones say KHX.... 8500....whatever means it is PC2-8500 RAM.
The 8500 RAM is from a forward generation compared to PC2-6400 RAM. You would suffer a performance hit assuming you are using the 8500 speed currently. I don't know if your computer recognizes it as 8500 or 6400, but if it does recognize it as 8500 and you put even 1 PC2-6400 RAM stick in there then it will recognize everything as PC2-6400 if it works at all.
Without more information I can't really tell you how much of a performance hit you would take, but I can tell you that I wouldn't do it.
I would try to find a stick that matches what I already have better than the one you suggested does.
OH lol I read some of that wrong. Yeah it wouldn be a bad idea to mix those sets because they are very different and would almost definitely not work. If it does work you will take a slight performance hit. Is there a good reason for you increasing your RAM capacity? If you don't need the extra RAM then you might as well save your money for something else.
The reason I'm interested in increasing my ram, is because I'm maxing my system out with some ram drives, and I used to be able to OC' my cpu to 3.1 when I had 4gb in dual channel , so I'm looking to fill the 4th dimm to have dual channel once again..the hyperx heatsinks turned out to be in the way of my cpu's fan (aftermarket) so I've moved the fan , and if I were to use pretty much any other dimm then the tall ones in there now, I could fit the stick right under the side mounted cpu fan.
So this is just research , as I'm looking more into getting a AM3 MB and using ddr3 memory ..as well as unlocking my tricore into a quad..+ overclocking it again!...
and again Ideally I'm looking for a MB with 2 pci-e x16 , x8 or x16 x16 , and capable or running ram around the 2k range as, the ram would be future proofed into the ivybridge platform
Also fyi running MSI cyclone 6950 (overclocked to 6970) and would love to get a second for crosspwnagefire.
Right now my cpu is taxed when I run bf3..and my ram pretty much is capped too, with a ram enhancer rocking my ssd read up to 2400 mb/sec on seq - sata 4 anyone!. Crystal Mark - 64bit.. although it appears my write speads might of been widdled down over time..as I can only write at 60mb/sec on this sata 2 ocz agility 2 60gb ssd.
You probably won't find a new radeon 6950 that can be turned into a 6970 because AMD changed the way they lock the cores from removing them in the BIOS to lasering them. Also, why would you get an AM3 build if you will be getting an Ivy system that comes out next year? I hope you also know that there won't be many Ivy board that supports 32 PCIe lanes for dual x16, they will basically have the same connectivity as sandy-bridge. Also know that unlocking an AMD CPU's locked cores fails more often than it works, I've read it has about a 40% chance of working.
Also know that you won't be able to overclock the CPU as much as you could with four cores as you can with three cores, suggesting your successful.
To be honest, I suggest you wait until Ivy comes out before you buy anything. It probably won't be much faster than Sandy but it can drive prices down.
You shouldn't mix RAM kits if you can avoid it, period. If you do they must be logically identical to get the highest chance of success. When it comes to overclocking your RAM significantly I don't just recommend that you don't mix, I tell you your chances of success are slim at best. If you want a 2000MHz+ overclock then you should aim for 2133MHz. You can either get a 2133MHz kit, a slightly slower kit and overclock it, or a low voltage kit and overclock it. The cheapest option right now would be to go for a 1.25v kit and overclock the hell out of it but that is also the most likely to be problematic.