1) it is not so much a question of high-end, but more of proper openGL support (zoom-in / zoom-out, other activities in the GUI rely on it) - most directX9 cards also have good openGL support, but with directX10 you're on the safe side (and the GUI will need less CPU) .... almost every PCIe video card is (eg. ATI 1950)
2) MIR is an application, so it can either run on the host or on the slave (needs just MIDI) - the intended setup is, that you don't need any additional VI/VE when running MIR, though you can if you like .... note: MIR outputs final audio mix stereo or 5.1 or 7.1 or ...
how many *instances of VI* (= instruments on the stage) you can run will depend on the performance of the used computer ...
3) you can use both as VI slave - however we strongly recommend to use the nehalem for MIR (nehalem = i7 or XEON 55xx), older processors might work up to a certain number of instruments on stage but are in fact out of the specs ...
4) yes ... the operating system will need ~1GB, the impulses can take up to 4GB, everything else (physical memory) can be used by MIR instruments.
note: 32 GB RAM on a MAC PRO nehalem means 8 x 4 GB means according to the triple memory bus design the upmost 8 GB can only be accessed as single channel memory and therefore might be accessible too slow in certain cases .... a serious mis-design of the current nehalem MacPros unfortunately (but don't worry too much - 19 GB RAM for sample headers should be sufficient for almost every arrangement ...)
5) yes, MIR requires a somehow modern ASIO capable audio interface - you could route audio back to your *recording host* via ADAT though.
please note: MIR is designed as integrated multi-impulse response reverbiation & mixing engine, it is not a *reverb* in the classical sense, mixing is already done within MIR by placing and tuning the instruments on the *stage*
hth, christian