I know Ubuntu is based on debian testing and unstable. I just want to be sure if current ubuntu packages will work on lenny. As i dont have access to a debian box now i ask here. Thanks in advance

Not unless you want to totally screw up your debian system. Never ever use ubuntu package in a debian system.

I repeat: never ever mix Ubuntu with Debian, unless you want to mess up your system thoroughly...

what about sidux?

sidux is an operating system based on the Linux kernel, Debian's most modern branch (called "Sid") and many free and open source applications.
sidux is 100% compatible with debian packages.

sidux contains only dfsg free software, you can add contrib/non-free support to your sources lists, and (#2) wget -O/etc/apt/sources.list.d/debian.list http://sidux.com/files/misc/sources.list.d/debian.list, and (#3) wget -O/etc/apt/sources.list.d/sidux.list http://sidux.com/files/misc/sources.list.d/sidux.list,

Default sidux only has sidux and debian sid repos. sidux repos basically just have the tweaked sidux kernel, useful scripts like smxi and the sidux artwork, and the occasional fix when a package in sid breaks.

Sometimes Ubuntu packages work ... You might get lucky ... What the heck, go for it. Reinstalling isn't really that big a deal, and you'll learn something. Newbies need that.

But they have about the same packages? I noticed Lenny now even has win64codecs etc. too. So why would you want to install Ubuntu debs? It is really not that hard compiling any missing packages yourself.