As bargainseeker said, I'm tempted to think that upgrading speakers will yield more benefit as far as sound quality goes. I would argue that you'll get more benefit having better main speakers for music listening and a matching center/rears for a solid 5.1 home theater system experience, than upgrading to a slightly better digital format that may or may not have DVD's available any time soon.

I don't know how much you have budgeted, or what gear you have now, but typically, speakers are the most expensive part of a system to replace, so this might influence your decision.

What kind of receiver do you have right now? If you plan on upgrading to 6.1 or 7.1 (I believe it's worthwhile, but only when you're happy with your 5.1 system) then a receiver with Pro-Logic IIx and the the latest DTS/Dolby Digital processing is probably a minimum.

A receiver might add more "fun factor" to your listening experience with the latest processing, DSP's, stereo modes (including direct mode, which I find helps a bit), provide more inputs, flexibility etc, but if you're more fussy about sound quality, I still think that speakers almost always offer the greatest area for improvement. Receivers offer slight improvements to the sound, speakers can offer significantly larger improvements to sound

What is it you like the least about your system right now? What do you use it for more, stereo music, or watching movies?