I have a JVC camcorder too.I have used this on both my PC's.The firewire connections are not from JVC and they both work fine.
Yes,it is worth getting a combination card,if you don't have USB2.If you also get a USB2 card reader for your digital stills camera the transfer rates will be greatly improved.
One word of caution,depending on your editing program,your camcorder may not automatically stop after capture.
With pinnacle studio 9+ all should be fine.
With Adobe you have to manually stop the JVC after capture.