on every other console I have ever owned its been possible but it seems the nes is a different beast!!! please tell me theres a cheap way to get better image quality outta this thing!!!
composite video, or Tim's NESRGB mod. That's pretty much it. Also, if you have a PAL console, you can't get 60hz from it unless you replace the CPU/PPU/crystal
The short answer is that when Ricoh originally designed the chips for Nintendo the ability to support 50Hz doesn't seem to have been even considered, so they came up with a very clever and low cost setup that generated everything from a single clock crystal and directly synthesized a NTSC composite video signal in the digital domain. The downside of this design was that it was completely tied to the NTSC video format so when they went to release the NES in PAL territories they had to design a different chipset to support PAL. They could, in theory, have designed some chips that had a mode pin allowing them to operate in both PAL and NTSC modes (as they did later for the Super Famicom), but they didn't.