The 5500 has two chipsets but only one is used by the mediator driver and so the RAM is reduced by a half, however you can still use 32mb (31mb video, 1mb audio) which is double that of any other card.
Speed wise from what I've read there isn’t much of a difference as only one chipset is used, but I think the 5500 is generally considered as marginally faster, with the added advantage of more usable onboard ram.
I have multiple screen modes set on my system with no problem :-)
