MS uPnP support is patchy, partly because the protocol is lacking in definition of some important things (transcoding).
Now myself i much prefer MCE 2005, its stable, but has one failing, it really dosen't seam to like the size of my music collection it takes about 10 seconds to load up. Not much time i suppose but it anoys the crap out of me.
Media portal is again, just not really good. If i'd made it, i'd be more forgiving of its failings, but it really just dosen't hit the spot. This is personal opion, try em all
As for spec, i'd recomend 2 gig of ram.