Memory clock is speed or data rate of the graphics memory. The graphics memory chips usually surround the GPU chip:
http://www.techreport.com/r.x/geforce-88…
Shader clock is the clock speed (gHz) of the numerous shaders or stream processors inside the GPU or graphics processor chip. The shaders are the green squares in blow-up diagram below:
http://www.techreport.com/r.x/geforce-88…
The other parts of the diagram form the GPU core that is clocked at a lower rate (mHz) than the shaders.
There is a direct correlation to how fast and how well the video card performs and how fast the shader clock is set.
http://www.anandtech.com/video/showdoc.aspx?i=2931&p=5
Helldiver:
I have owned a GTX 285 OC, and ATI HD4870, and ATI HD4850, two 8800GTs, one GTX+9800. I have worked with Intel engineers, ATI engineers, and Nvidia engineers and all have said the same thing about FSX, but I can't repeat it here. So to keep it polite I simply say that it was not Microsoft's shining hour when they made it. However I love it compared to other flight sims most of the time!
I have tested over 50 various video cards at TechCorp in the last six months and here is the conclusion I came up with based on the above paragraph and my testing:
If you have a quad core or and i7 Core get a card like a GTX260 or GTX 275 for the bandwidth and clock speed you will get amazing background textures if you set it up like Nick Needham has his. Don't be over agressive on the eye candy at first-inch your way up little by little and use nHancer even though the newer drivers include better AA.
If you have a Core 2 Duo (make sure it is an E8400 or faster) stick with your 8800GT and load Windows 7 64bit OS on your system and buy fast RAM.
If you just have to upgrade the 9800GTX+ 512 MB is just fine-get the fastest you can afford. If you have a Intel board and are not interested in running SLI some day the HIS ICE Q4 HD4850 Turbo is nice and so is the Sapphire HD4890. I would not waste my time on the HD4870 for now since the HD4890 is out there if you want ATI. Be careful trusting reviews on the Internet some people just have an axe to grind and skew the results just to prove a point-no names mentioned.
I've been testing all day and I'm tired I hope I type all this correctly I will correct any mistakes tomorrow night.
Ted