[CentOS] imaging a drive with dd

I am building a mailserver and with all the steps, I want to image the 
drive at various 'checkpoints' so I can go back and redo from a 
particular point.  The image is currently only 4GB on a 120GB drive.  
Fdisk reports:

Disk /dev/sdb: 111.8 GiB, 120034124288 bytes, 234441649 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x0000c89d

Device     Boot   Start     End Sectors  Size Id Type
/dev/sdb1          2048 1026047 1024000  500M 83 Linux
/dev/sdb2       1026048 2074623 1048576  512M 82 Linux swap / Solaris
/dev/sdb3       2074624 6268927 4194304    2G 83 Linux


and parted:

Model: Kingston SNA-DC/U (scsi)
Disk /dev/sdb: 120GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Disk Flags:

Number  Start   End     Size    Type     File system     Flags
  1      1049kB  525MB   524MB   primary  ext3
  2      525MB   1062MB  537MB   primary  linux-swap(v1)
  3      1062MB  3210MB  2147MB  primary  ext4

what dd params work?

dd if=/dev/sdb of=os.img bs=1M count=3210

?

thanks