Mostly writing notes so I don't have to go through this again, but I'm sure someone can benefit from this.
Before - Dell Optiplex 755 with 10.5.8 Vanilla
1 Visible HFS+ partition, 160GB Samsung Disk.
Wanted to upgrade to larger 640GB WD Harddisk. That task was simple, doing a sector for sector copy with freeware called Easeus Disk Copy (Bootable CD, 640GB drive hooked to internal hidden 2nd SATA port and powered by external SATA power adapter).http://www.easeus.com/disk-copy/
It created an exact duplicate, and booted just fine. However, you could not add a partition, or resize with just about anything, even tried iPartition Boot CD.
The only way around this, was to boot a minipe disk (google it), go to paragon hard disk manager, and delete a 100mb or so partition that was hidden, after the main partition. Merge the free space at the end, and reboot into OS X.
Now you can resize the GUID 1st partition, or at least make a 2nd partition on the new drive. I chose to make a 2nd partition, and it just worked. I don't know what the 100mb partition at the end was, but since I still had the original disk, and was working off a clone, didn't really matter if it didn't work. I highly recommend you doing the same, unless you don't care about the install.
To backup, I just use an identical 640GB drive, that I occasionally hook up and copy with the Easeus Disk Copy boot CD. Additional note, you don't have to copy all 640GB if you only had a 160GB partition and some free space. You can cancel the copy about 27-30% into it, and it will still work fine (the Disk Copy is sector by sector, so it's slow, this will save you some time).