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.