For the old bike I would say make sure the spokes are tight, change the saddle. Ride it where you can, but mostly save your pennies and get a new bike. In the 10 years since that bike has come out a LOT of improvements have taken place in mountain bike products. Geometry, components, materials, pretty much everything. The money you put into that old bike will not allow you to take advantage of everything out there and delay you from getting into a more recent bike.
BUT, if that is the bike you have, for sure ride that bike. The bike may limit some of what you can tackle, but you can ride. When you finally can upgrade you will be blown away by the capabilities of new bikes.