my opinion:
the RAM usage of- for instance firefox- is the same if you use it in lubuntu/xubuntu/mate because it's the usage of firefox itself. (ab big part here are add-ons and the number of open tabs...
Use less addons. Use not so many tabs - it will help. btw- have you ever really testet without flash - meanwhile most pages are working.
The difference of the kinds of *buntus like Xubuntu, Lubuntu, Mate is in my opinion not as big as allways thougt. Xubuntu/mate are about the same, Lubuntu needs a little bit RAM lesser.
I have an older notebook (1 Gb RAM) which I upgraded from xubuntu to Mate without significant performance changes.
Before Mate i used xubuntu allways on older machines. But: i hate thunar (and loved nemo)
I don' like the menue of xubuntu ( and Lubuntu)
... So i allways had about a dozen things to change after installing, an in sumn they were less consistent.
Mate is a round thing > I liked it from the beginning. A clear structure and menue, caja is way better than thunar, and so on...
partitioning:
Every harddisk in an older PC is able to host a minimum of 4 partitions. Either 4 primary Partitions (dos, windows, linux and other systems are bootable from primary partitions)
OR 3 primary Partitions plus an EXTENDED Partition whitch can host numerous logical partitions inside itself. Linux is able to start from here (inner Partitions)
Use either 1 partition (plus swap!) for komplete Linux, or things like: a root Partition (10...40 GB) for the system + a swap (i would use about 2 GB with your PC) + the rest for /home (overall 3 Partitions)
the swap area is the key for systems with little RAM
You are not able to do so? Use a live-CD(or USBstick, install gparted (sudo apt-get install gparted) unmount in case of used partitions (for instance if you searched datas with a filemanager and it mouted some things automaticaly)
Delete/create partitions as you want.
If you boot your installed system, you may not be able to unmount/delete Partitions in use)
backup: its not really a good idea to make backups to another partition of the same disk: if the disk dies, the backup is also dead.
If your /home is an extra partition you will be able to do an new install without loosing your data anyway. (partitioning step: say "something else"(?) make a new /root and swap and than: use /home with mountpoint /home without formating...)
hope it helps...