Can I use 3 GB RAM with AMD motherboard?

Individual RAM sizes will be in powers of 2 :

512 MB,1GB,2GB,4GB...etc.

However, you can pair them in your RAM slots in any manner.

For Eg. 1Gb + 512 MB = 1.5 GB is also possible

Just make sure your motherboard supports the amount of memory you put in. Usually, this is something like "Upto 8 GB Supported". So you can't put in 16 GB!!! (even if you could afford it)

It depends on the model of motherboard you have, there are ussually 2-4 available slots for you to stick memory into. And your motherboard will also have a limit to the amount of ram ( memory ) it can support. So if you want to find out how much ram your computer can support you should look at it for a logo, and a model number and then search it on google. Most motherboards produced in the last year can support 4 - 8 gigabytes of memory.

Yes 1100 watts is more than enough for all but a few computers. Mine is running just fine on 600. The crosshair series does have crossfire and sli capabilties so you're good for dual video cards. Make sure that the graphics card and cpu are compatible with the motherboard and you're good to go.

Ram is produced in the sizes you mentioned (1, 2, 4, etc.) but of course you can combine them in whatever way you want.

You said it yourself. You can ADD ram like 1 or 2 or etc, but whatever combination, as long as the specs match your computer, will work. =)