We specialise in osCommerce as well as Magento and this has meant that we have helped a lot of osCommerce stores upgrade to Magento.
The process can vary from fairly to painless to a bit more complex
One of the key things is that if you do a direct migration, you will miss out on a lot of the cool catalogue features of Magento. For this reason its definitely worth thinking about reorganising your catalogue, using things like configurable products etc rather than just simple products only.
Speed is an issue, the fact is that osCommerce is generally faster than Magento on the same hardware. The reason being is its a lot less powerful and complex. Use the right hosting company though and the difference shouldn’t be too great. The increased power is definitely worth it.