Isso é da análise da AnandTech da AMD Radeon HD 5450:
Finally, since the 5450 is a low-profile card, we have yet another variant in the port configuration. Our sample card is equipped with 1 DVI port and 1 DisplayPort directly on the card, and a VGA port on the bracket attached to the card via cable. The 5450 – like all other 5000-series cards – supports 3-monitor Eyefinity, so all 3 ports can be used at once, although the VGA port is obviously going to make it less desirable here.
We should note that only cards with a DisplayPort will support Eyefinity, which means that if a vendor decides to equip the card with an HDMI port instead, they have to give up Eyefinity to do so. We expect to see some cards with HDMI for the HTPC crowd, but at this point we don’t have any idea as to how many there will be. DVI-to-HDMI adaptors still work here, so we may see vendors spend a bit more money and go that route instead.
Eu destaquei a parte em que você está interessado. Basicamente, sem o DisplayPort, não há Eyefinity e, portanto, não há suporte para três monitores.