As far as I know - as long as the target addresses are available via both cards then it should share the work - however in my experience Windows XP doesn't take much advantage of this - there certainly isn't any tick box to select which card is used by which program...
I believe the "Server" versions of Windows are better at handling more than one network card, but this is more to do with the programs run on it (e.g. Mail servers, or FTP servers) than the operating system alone.
You can test the "throughput" of the two cards by opening "Task manager" and switching to the "networking" tab... All your network cards will be listed. So if you go and do something on the network; you can monitor how windows is coping. I think you'll find it'll bias the wired connection as this has the greater throughput.
Bear in mind that 100Mb should be more than enough for most day to day apps (You shouldn't really see it peak to 100% useage); so don't feel hard done by it. :-)
All the best