I have 11.0 installed in a 486 with 16 MB of RAM. I can't run X on that machine, but otherwise 11.0 functions.
Everything is horribly slow because of the lack of RAM and swap doesn't really help. WFWG 3.11 on that machine flies.
Keeping old computers running is a hobby for several Slackers here. Generally, to tinker with older hardware means some sacrifices and often means not using the latest software. If you are using this older laptop for tinkering then be bold and experiment to your heart's content. If you are using this machine as a production system for everyday use, then you have to accept some realities and limitations.
I don't know the minimum RAM requirements for later versions of Xfce. Perhaps verify those requirements before proceeding down a path that might waste your time.
Yet 64 MB of RAM will test the patience of anybody wanting to run the latest desktop environments, even Xfce. You're facing a steep uphill journey.
A window manager environment will help but modern apps are greedy for RAM.
As a quick go-no-go test, I booted a spare Pentium II machine that has Slackware 12.2 installed. 12.2 came with Xfce 4.4.3. To simulate a 64 MB system I booted the system with the kernel boot option of mem=64M. 12.2 booted into run level 3 to a login prompt. At that point the system was already using swap, but I did not reconfigure startup scripts or processes. I was able to start Xfce. The desktop was sluggish and "free" showed 28 MB of swap being used. In other words, with no apps running, my quick test was using just under 100 MB of RAM with only a terminal window open.
Repeating the same 64MB simulation with IceWM and no apps running used about 144 bytes of swap. Xfce 4.6 will be more hungry for RAM than 4.4.x and so will any modern apps.
As mentioned, if you decide to stay with 11.0 then you will have to update many packages and recompile Xfce too.
With that said, if Xfce 4.6 will run on 64 MB of RAM, do you have access to another computer? In the past when I faced RAM limitations such as you describe that prohibit installations but not running the operating system, I temporarily moved the hard drive from the older machine to a newer machine. I installed the operating system there.
If other hard drives are connected in the second machine while I installed the operating system to the old hard drive, I edited fstab on the old hard drive as necessary for the correct locations in the old machine. Then I returned the hard drive to the old machine.
If you find another 64 MB of RAM for that machine, you are going to have more flexibility trying to update Xfce to 4.6, which probably requires Slackware 13.1 or 13.37 anyway. You still might need to temporarily move the hard drive to a different machine to perform the installation.
Hopefully you have some fun playing with this old hardware.