If the movies are important, then I wouldn't recommend a1200. There isn't software support for hardware decoders on Voodoo or anything else and 603e isn't quite up for the job with cpu, at least with heavier avi's etc.
For those other tasks expanded a1200 will work fine (as amiga system, lets not compare browsers to pc browsers now). Although cd-burning is bit slower etc. But expanding the a1200 is always bit hacky and new generation computers are much cleaner solutions and more stable in long run. I'd really recommend SCSI, if you go a1200 way. OS is running on 68k cpu and SCSI makes it more fluent to use.
As for compatibility, system friendly software will most likely to work, but real a1200 is best for most games and demos etc, which uses a1200 hardware directly. Older games must be emulated with uae or similar on new systems.
If you make that upgrade to your a1200, you probably end up with two monitor system... one for voodoo and one for aga modes. Unless you make some kind of scandoubler hack or something to get different modes to same monitor
In any case, programs can open aga or rtg screens and you see them if you have correct outputs connected
One way is to buy tv-card for Mediator and connect video out from amiga to it and then watch aga modes from Voodoo with tv-software. It works with non-multitasking programs too. TV-card keeps feeding picture directly to Voodoo mem without using cpu. I've made little script to start aga programs... it starts tv-software, makes it fullscreen and then executes the actual aga software. Picture quality is worse with tv-card than other ways though.
If movies are more important than running 060 needing demos/games for you, then I'd say wait for A1 or get Pegasos2 (you can get complete systems from 700e). Overall speed of applications and movie watching is far more better with next generation hw. You can still keep the old a1200/030 for games etc
