You want to learn, cool! Here's a run-down on a 2-way crossover in the PEQ. You may have to use different channels than the Sub and Center I use in the example. It depends on how your soundcard routes it, although I'd bet your card would "see" MC's Sub and Center channels as 3 and 4.
The main thing in doing a crossover with J River is getting the channels routed properly. Go to the DSP Studio-Output Format. Select the number of channels that you need from the drop-down menu on the right. It sounds like you're doing 2-ways so 4 channels will work. Make sure "Mixing" is set to no up mixing or down mixing.
One thing you need to understand, is that the different sub-menus of the DSP Studio that are listed on the left can be moved up and down in order (except for Output Format and Volume Leveling, these are locked). This is important because these are processed in order from top to bottom. If it isn't already third from the top, move the Parametric Equalizer up under Volume Leveling. Then in the PEQ click Commands/Add/Mix Channels. This will allow you to tell JR which channel goes where, because until you tell it otherwise it only sends the signal to L & R. After you click "Mix Channels" choose Left for the "Source", and Center for the "Destination." For "Mode" select Copy. This will copy the Left signal to the Center channel. Now click "Commands" and add another mix channel filter, you'll see another one added to the list of filters. For this one choose Right for the "Source" and Subwoofer for the "Destination." Now you have 2 channels with a left signal and 2 with a right. I'd use the Left channel for your tweeter/mid and the Center channel for your woofer. Your Right channel would then be your other tweeter/mid and the Sub channel would be for your other woofer.
From there you can use whatever filters you like from the Commands button to do your eq/crossover, making sure you assign the filters to the correct channel. The filter list works like the DSP Studio sub menus in that it works in order from top down. You can drag filters up and down in the list. Make sure that any eq/crossover filters are below the channel mix filters in the list. Any filters you put ahead of the channel mix filters will be done globally to the L & R channels, which will affect all 4 channels.
In order to do measurements processed through MC you'll have to use the Loopback feature. This thread:
http://yabb.jriver.com/interact/index.php?topic=70242.0 will tell you how to do that.
As far as soundcards go there's a bunch of PCI cards geared to gamers that should work. I use a 12-channel pro interface, but that's a good bit more expensive then a gamer card. Depends on what you want to spend. I'd wait and see if you can get it to work with what you've got first.....