It all depends on what sort of things you value on this front, and then, how much you value them.
In terms of picture quality on an LCD display, there's no real comparing of SVGA and DVI. DVI is the clear winner since it's a digital connection system, so there's no need for any signal conversions, where unavoidable quality loss occurs.
On the audio front, the same is true. If you're using analog connections now, you will find a big improvement in audio quality if you go with a SPDIF passthrough setup. Particularly if you have a 5.1 speaker setup. There's no comparing upmixed stereo to true 5.1 discreet channeling. The clarity of the LFE channel alone will do wonders for sound quality.
Now, the question is, how much money these things are worth to you, which is something only you can answer. It's also not something that requires an all new computer. You could just replace the video card with one that supports DVI and the sound card with one that has SPDIF output.
In my personal opinion, if you already have a 30" TV and home theater system, then you strike me as the sort of person who would consider it worth the money to have these things, but it doesn't require a whole new computer.