Allow me to explain the slightly bonkers title. I'm building a new P55 machine, and while 8GB of RAM is just outside of my budget, I've determined that 4GB of RAM won't really be enough either (I do pretty heavy photo editing, and will be running Win 7 x64).
So I'm wondering if I can combine a 2x2GB kit with a 2x1GB kit for a total of 6GB of RAM.
Just get 4 GB and upgrade to 8 GB down the road... or just take the jump and get 8 GB now. If 6 GB is truly the right amount of RAM for you NOW... then a little ways down the road you'll need more... and you'll have used all 4 slots. You're trying to go against the flow on this one bro. Don't.
I have the same mindset as Rodney. Nothing worse than buying something that you pull out in a few months and isnt reuseable. Low density memry has almost no use at all. Win 7 x64 will use all of that 4G of memory unlike the 32 bit version. Various benchmarks on this site also show that the fastest memory doesnt deliver huge performance gains either. So if 8G is what you need, trade away a little speed and get the slower RAM to get to 8G.
You can use different RAM in the 2 pairs of slots without impact or stability issues. Its always best to keep them similar to reduce the number of issues you have to deal with in case something does goes wrong. The memory must be the same within the slot pairs, the 2 Slot 0's or A's, etc.
One side effect is when populating the second bank, the memory almost always slows down. This is because of the extra load, they change the Command rate from 1T to 2T. You can see this in CPUz (not all do it but most do). Thats also why I said get the slower RAM.