I can see the benefit of having an 8x card if you plan on doing some serious 3D gaming on your MD (I've run C&C 3 on my Hybrid). If you're not looking for gaming then there really isn't a point in using the more expensive card as LinuxMCE itself doesn't use the extra horsepower.
The 8x cards actually work just fine. I believe the AVWizard sets it to use the nv driver instead of the nvidia driver, however. If you edit xorg.conf file to use the nvidia driver instead and make sure you set the proper output if you device has multiple outputs, you'll be fine... Until this (really minor) problem is fixed I wouldn't recommend anyone who doesn't want to get there hands dirty to use a new card...
Now that doesn't mean your TV is compatible with whatever mode you are trying to throw at it... This is actually the major cause of most issues, even with the people using the 8x cards. Editing modelines to use nonstandard timings so a specific TV (that uses non-standard timings) can run a higher resolution is much more complicated than changing from the nv driver to nvidia...