If you plan to upgrade from Magento 1.0 to 1.4, be prepared for some heavy lifting. Just finished a client upgrade who didnt do any upgrades for over 2 years. Not doing that ever again.
Might even stick with 1.3, as many of the old modules behave badly under 1.4.
Magento is a massive solution, but community support via Varien sucks big time. So anybody that is not ready to dig into code, should use another solution or find someone that can help.
Cheers