The extra 100 Mhz won’t make that much difference, now or in the future. By itself, I’d probably use the extra $200 elsewhere. Combined with the DVD, however, it’s more than worth it. Consider that it’d be nearly impossible for you to go buy the DVD card and drive for $200, and there’s a real chance that you’ll find it useful or necessary sometime during your academic career (say, when you end up writing a paper on Spinal Tap and need access to the additional material only on the DVD edition).
If, however, you end up choosing between the DVD/additional speed on the one hand and the additional 128MB RAM on the other, go for the RAM. Go for the RAM in any case. Do not expect that 64MB will be enough for the things you want to do now, much less two years from now. I have a 266 Mhz iMac that I bumped up to 96 MB immediately on purchasing it (as high as you could go at the time without having the RAM professionally installed). On many occasions in the last 18 months I’ve wished I had additional RAM available; only once or twice has it made it impossible to do something, but in many cases it’s been much harder than it would have been with more headroom. Having only 64MB, it wouldn’t be long before you’d find it being a significant limitation.
And I apologize for the empty post before this one. I inadvertently hit the submit reply button {sigh}.
Heck yes, they’re definitely worth it! Like rackensack said, you never know when the DVD-playing capability will come in handy.
There’s always room for RAM. 
Seriously, though, go for it. Adding more RAM is one of the best ways to improve your 'puter’s performance.
If you can wait a little bit, catalog sellers should be offering free RAM deals soon. I got 64 MB free with mine.