I have a u8668d motherboard with a 1.8 ghz celeron processor.
The MB calls for DDR SDRAM memory. PC2100 to be specific.
My question is does the memory sticks have to match for this? Could I have a stick of 1 ghz and a stick of 512 in the othe slot? The system seems to lag in some games. especially when there is a game played that has good complex graphics.
Also could I use pc 2700 or higher RAM. I was told I could but couldn't get the higher speeds.

Yes, you can mix diferent sizes of RAM sticks. Yes, faster DDR will happily run at slower speeds (in theory.. you may have to shuffle them around in slots and mess with BIOS).

More RAM would ertainly help, but I would say you biggest problem is that Celeron there..

I forgot to say that I have 2 sticks of 256 in it now. I want to buy a 1 gig stick and run it with i stck of 256.

It seems that I can get faster RAM cheaper. pc2100 is more $ than the faster?

In general, you can do it, but it is not recommended, especially with faster boards and faster Ram.
The FAQs section on ram will answer your questions.
http://www.techspot.com/vb/topic47210.html