How much is the WiFi card you found? The mid level x30 is only about $50 more than the low level one. I can't imagine that it would be cheaper.
Remember also that not only do you get WiFi, but also BT, both of which can be used simultaneously in the mid and high level x30 models. That is not possible to do on yours since you can only have one SD card at a time. And the memory on the low is only 32 RAM/32 ROM as compared with 64/64 on the other two. That's why you're gonna have the price difference
Is the 1940 better than the low x30? Depends on what you need. The 1940 has an older and slower processor but the x30 has less memory and no built in BT (which the 1940 has). The 1940 does look a little more stylish, but does that matter? The x30 has WM2k3se and the 1940 only has WM2k3. I think that's a big advantage since you now have WMP10 available to you.
And the x30 was $100 less than the 1940. They're different...I doubt anyone can really make a case that one is significantly better than the other. (though if you're still within your 30 day period, you may wish to exchange it for a mid level x30...that just seems to be a lot cheaper than buying a WiFi SD card and you get so much more...)