Like Bob says, that's a fair old ride to complete in two days. I rode Ushuaia-Rio Grande which was half good dirt and half tarmac in '04 and stayed at Hotel Argentina, San Martin 64. It's a great place for bike travellers, parking under cover out the back, really good breakfasts and a party atmosphere in the evenings. Next leg Rio Grande-Porvenir (fantastic road) where I stayed at the XXX Video Store Hospedaje; cheapest digs in town.
If you're planning to come back on the Melinka from Porvenir-Punta Arenas make sure that it's sailing on the day you want. There's some rule about no trip on the last Tuesday or Wednesday of the month. Also bear in mind it takes three hours. Although I didn't use it, the Primera Angostura ferry would probably suit you better as it only takes 30mins (I think) and sails about six times a day. The road once you're back on the mainland is excellent all the way to Puerto Natales. There's quite a bit of rubber stamping and form filling before you board the Navimag, so leave plenty of time for that. Don't miss the "Happy Sour" each day when cut price Pisco flows freely and make sure you pack your motion sickness pills, that tub can roll!
There are also HU Communities in both Puerto Natales and Rio Grande that you can contact nearer the time for more info.