Linux distributions that are easy to Upgrade
 

We are looking for ditrubution that is easy to upgrade in future (for long term)

It should have wireless and gfx drivers pre-installed and should be pretty stable.

We've been looking at Sabayon but it failed to boot in short term (after upgrading) as it uses cutting edge packages.

We know that installing drivers can be painful task, but maybe there is distro that is easy to upgrade (dist-upgrade, rolling release, etc.), has drivers and can be changed to stable packages easily. Thanks.

Ubuntu is a nice easy distro. Here are instructions to upgrade: http://www.ubuntu.com/download/desktop/upgrade

For stability I recommend the Long Term Support (LTS) releases, which come out every 2 years and are supported for 5.

Indeed, but it doesn't come with wireless drivers pre-installed :P

Of course it does.

Is there a specific piece of wireless hardware that's giving you trouble?

only open source drivers are pre-installed on ubuntu. Os it now possible to easily install non-free drivers via "System>Administation>Additional Drivers". It didnt work last time i tried Ubuntu (atound 9.04).

If you tell us your wireless chipset, we can help you get it working. Personally I wouldn't choose a distro based on a 5-minute procedure to install "non free" firmware/drivers. If you limit yourself to distros that provide this (maybe Linux Mint?) you will miss out on most of the best distros (Ubuntu, Debian, Slackware, Fedora, etc.) which adhere to the "open source" philosophy.

You can always avoid upgrades with a rolling-release distro. Not a bleeding-edge one like Sabayon, but something stable like Fuduntu. I think that's also good for wireless (although I haven't tested that aspect personally).

I had success with Ubuntu's "Additional Drivers" feature with Broadcom's drivers since at least v. 9.04, the only non-free drivers I have experience with. I've also been able to install them with Mint, SalixOS, and Slackware (though the Slackware experience was a little more involved because I had to track down three packages--firmware, firmware cutter, and driver). Those are the distros I have direct experience with.

Debian.
It's oldstable to stable dist-upgrades are the most reliable you will find.
Any wireless not working ootb is pretty much a 5 min trip to wiki.debian.org away, and if that overrides your requirement for upgradeability for years then your priorities are a bit muddled.

I have a box that started life as Etch, has migrated through Lenny to Squeeze and is about to become Wheezy. It has also moved mb & hdd a couple of times and hasn't missed a beat.

My choices would be Slackware and Debian. Perhaps OpenSUSE.