The EeePC comes with Xandros (Linux), and as you suggest it’s an old crippled Linux disribution… pretty much ALL Linux distributions will run on an EeePC 900.
Maybe you should look at some of the many Ubuntu based distributions that were specifically made for the EeePC, such as:
EasyPeasy:
or
Leeenux
http://www.leeenux-linux.com/
Both of the above are based on Ubuntu 10.04 LTS (netbook edition)… so you there’s nothing stopping you from going for Ubuntu 10.04 LTS Netbook Edition, which can be downloaded from here:
http://releases.ubuntu.com/lucid/
Or you could try the latest Ubuntu 11.04, which may or may not be slower on your hardware, as 11.04 isn’t specifically designed for netbooks.
http://www.ubuntu.com/
There’s a test of 11.04 on an EeePC 900 here:
http://musante.i.ph/blogs/musante/?p=92
(the author seems to prefer 10.04 UNE, but that’s a personal opinion)
Any of the above will give you a MUCH larger choice of software, and they are all still under active development, so are still receiving updates.
Why not try them ALL ?
They are all LiveCD’s, so can be installed to a USB stick… this will allow you to test drive them without affecting the current Xandros OS… once you find the one you like, install it to the SSD, overwriting Xandros.
Be aware when running any Linux distro from a USB stick, it will be slower than running it from the SSD, so don’t draw any performance conclusions from a LiveUSB.
If you need further info on how to create a LiveUSB stick, just ask 
It doesn’t have to be an Ubuntu based distro, there are plenty of others… many of which will be faster than the Ubuntu based distros, but may be harder to set up or find help for, or may have less software available in the repositories.
So it really depends on your priorities.
My advice, if you’re new to Linux… go for one of the Ubuntu based distro’s… ie. ANY of the above 