This is my current Setup
43GB NTFS for windows, and in a logical partition:
1.5GB partition for swap
18GB reiserfs partition for all of '/'

This is my plan.
Going to use qparted live CD to:
1) Create a reiserfs partition on my USB HDD
2) Transfer everything from my only partition '/' onto this new partiton.
3) Delete the logical partition of the HDD
4) Size down the NTFS
5) Create a new logical partition on the HDD
6) Create a 1GB swap
7) Create a 20GB reiserfs for '/'
8) Transfer everything back from the temp partition I made in my USB HDD

Concerns:
a) Should everything work like that? do i need to edit fstab or anything else?
b) Will grub work as it is (on my MBR)? I suppose it should because the drive defintions should remain the same since I will create the partitions in the same order
c) Anything I didn't concider?

I am not sure, but I think stage 1 of GRUB (the part on the MBR) stores the physical location on the disk of stage 2, which will have moved in this operation.

Check your /etc/fstab file to see if refers to the partitions by number or by label. If they are referred to by number you should be OK if you put your swap and root partition (/) on the same partition numbers they were originally.

In any event, if I were you, I would make sure I had a floppy, flash drive, or something that I could boot into a GRUB command line, and verify it works before I altered the HD in any way. This way regardless whatever happens, you should be able to boot your system to recover, if need be.

will it preserve the file rights this way?

To preserve ownership info you will need to do both transfers as root. Then using the -a option with either cp or rsync will preserve everything.

EDIT: Again, before you alter your HD, I suggest you check the contents of your USB drive. If you are planning on booting from the USB to transfer things back, verify you can do that to.