Better forget about it. The CPU is puny and the memory is way far from sufficient (
800 MHz Marvell 88F6281 (Kirkwood), 128MB RAM).
If you want to try anyway, install Debian Squeeze on your NAS (
link; installation instructions inside) and try LXC; install it with
apt-get install lxc (
guide - you'll have to improvise, since the guide is for a different NAS, namely QNAP TS-219P+, which has a Marvell 6282 1.6GHz CPU and 512MB RAM, and probably a newer and better kernel).
Please remember that I can't guarantee that it will work. If you just want to extend your NAS capabilities, limit yourself at using Debian Squeeze. The NAS' kernel is rather old, so you can't install Wheezy.
If you desperately want Ubuntu, you'll be limited to Jaunty, the last one compatible with armv5. If that's the case, PM me. I'll try to help you install it as chroot (the same way D-Link did with Debian Squeeze).