Captured DV footage always looks heaps darker on a computer monitor than a TV. For this reason, it's always a good idea to output to a TV monitor while editing. I'm not sure if you can do this with moviemaker (anyone?).

You definitely should get a firewire card as well, because you'll be losing some quality capturing with USB (you also wont be able to output to a TV as easily using the USB as if you use the firewire connection). You only need the most basic firewire card, although, you might want to have a look at the possibility of getting a package that includes better software than moviemaker (or hardware acceleration).