If you go down the lens filter route then the camera needs to be on a tripod as you're looking at exposure times or 20 seconds plus, with a converted camera you can shoot handheld.
It's not really a DIY conversion job on a D300 (unless you're really confident with a soldering iron), so budget around £300 to get it converted properly.
towershot said:I cannot remember now,but sure I read the D70 was easily converted to shoot IR :shrug:
I would not waste the D300.
Have a read.
http://www.lifepixel.com/tutorials/infrared-diy-tutorials/nikon-d70-d70s