Bob has it right. Toast7 or Popcorn will automagically compress
and burn. With awfully good video results.
Before you even think about a DL disc (and use the +R persuasion
for best compatibility) see if your drive can use it. Go to System
Profiler and look under the ATA section. That'll tell you the brand
and model of superdrive you have. Then check the manufacturer's
site or videohelp.com (DVD Writers tab on the left) and see what
the beast can deal with.
Note: in videohelp, don't search on "matshita." Use "Panasonic"
or just the model number, say "835." The 835 does do DLs.