Will my mobo support a SSD?

Yes, it has sata ports.

Yes, if you have available SATA ports you can use an SSD in there, no problem

Thanks for quick replay.

But I know that SSDs are SATA3 6Gb/s

And I checked the spec of my mobo, on the storage tab.
http://www.asus.com/Motherboard/M4A785M/#specifications

Southbridge
1 xUltraDMA 133/100/66
6 xSATA 3 Gb/s ports Support RAID 0,1,10,JBOD

So my mobo only support up to 3Gb/s? does it effect any performance if I get a 6Gb/s SSD connect it into a 3Gb/s port?

Not all SSD's are SATA III not that it matters, if you get a SATA III SSD, you'll simply be limited to SATA II speeds, everything else will be the same.

It just supports up to up to 3Gb/s, like said above, you'll run at sata II speeds, so it wil affect the performance of the ssd, if it's a really fast one(under heavy loads), but still it's better that not having one, it won't degrade that much.