Yeah, you're right. I think eventually I'll probably buy an ATX case and MOBO and migrate my components over, it wont be for a while as my computer is brand new and can handle most things very well at the moment.
The components themselves are good and can be great with some choice mods, it's just the BTX MOBO holidng it back.
I really hope I can find an empty XPS 730 case on Ebay one day, I'd gladly buy a new MOBO and upgrade then
I still think the XPS 420 is a decent computer though although, like you say, the BTX motherboard limits it a bit (though you can still change a lot of stuff in it).
At the time the XPS 420 was cheaper then building my own PC. I'd still probably buy a Dell in the future if the price was right though, of course, you have more control over a home-build.
For anybody thinking about changing their XPS 420 PSU, I can say that the Coolermaster 700W Silent Pro Modular also works in the XPS 420, but it's not available in America at this moment in time. It also comes in 500W and 600W flavours too.