Difference betwen single and dual rank memory?

[Clockwork_Killa]

Can anyone tell me what the difference between single and dual rank mexmory is? I need to get some more memory for my current build, so far I have two 512 x8 single rank I was going to get a 4gb kit but my only options are either a single rank x4 kit or a dual rank x8 kit. I know you cant mix x4 and x8 in the same bank, but is their any real difference between single and dual rank and can you mix the two?
Thanks

[Beowulf1100]

If you mix Dual and Single channel memory.
The dual will act as a single channel module.

[Ellis Alexander]

Each motherboard can only support so many rank selectors. So once you
have used them up it will not see any more memory even if it is present.

There are 3 sets of dimms on the 1425 and 2850. You can do mix and
match but the largest amount of memory that is possible is 4 x 4 gig
dual rank dims for a max of 16 gig. I've not seen anybody whom has made
a single rank 4 gig Dimm.

[Clockwork_Killa]

Well I will be running two processors, so I would only be installing one of each memory module in each bank, so I would have 1 512 single rank x8 module and 1 4gig dual rank x8 module in each.
Would that be alright? If not I guess I can always just install the 4 gig modules and get rid of the 512s.