Why don't you take the slower stick out? 4GB of 800 or 1066Mhz in Dual Channel should be plenty, if the third stick is that lowly clocked.
There's definitely no point buying different RAM if your best stuff is at least 800Mhz (check with CPU-Z
(Memory tab, DRAM frequency> x2 equals RAM Speed as you probably know). DDR2 is very high in price, considering the frequencies and timings of it's DDR3 counterparts today, might aswell save new RAM for a new Mobo.
As said, a cheap 955 C3 will do you great, or if you fancy your luck with the 560, by all means go for it. There's a $45 difference between the two brand new.
I don't recommend getting an X6 with that Mobo, I've seen a few problems regarding them with X6's, likely due to the lack of MOSFET cooling like everything else that has a problem with the X6's. 8)
If your Rep were higher, you could see if anyone here at the marketplace is flogging anything up that alley, if you wait a little longer, you'll likely see plenty of these up for grabs as people sell up for SandyBridge and Bulldozer replacements.
Have a good butchers over the next few weeks and see if a good deal pops up somewhere.