Memory Not Allocating Correctly - Crashing On Windows 7 64bit

I've had 4gigs of RAM installed for very long and I was able to work fine with my browser open + Photoshop and Blender in Win XP. I've quite recently upgraded from Win Xp 32bit to Win 7 64bit. After installing Win 7 64bit I added 2x1GB sticks into my system to take advantage of memory I had available. These "new" sticks were of lower clocks than my 2x2GB I had installed so I tested clocking them higher. Well that wasn't really stable so I figured running them at lower clocks wouldn't hurt all that much.

So everything seemed to work fine, Memtest didn't whine about anything and everything seems to work well enough. Now however I can't have my browser + the other programs open at the same time but windows keeps whining about it running out of memory even though the taskbar memory amounts don't add up at all. I have like 5Gigs used but adding everything doesn't even get up to 2Gigs. I know about windows keeping unused memory ready for use but it doesn't seem to be that or otherwise I don't know why my programs keep crashing all the time.

I have a load of tabs open in my browser but they don't take all that much memory. And I don't have any other programs open that would use that much for anything. My display turns black sometimes and sometimes (not every time) after that I get the display driver error and that it's recovered. I've had this very seldom before but now it seems quite common so I figured it might have something to do with that the display driver doesn't get enough memory to use either. Just a wild guess but textureloading in Blender doesn't work all the time etc. so it seems like a vRAM problem to me.