Six months will be here before you know it.
I would upgrade everything (receiver & HTPC) at the same time.
CPU, RAM, MoBo, HDD -- already have.
Gotta have HDMI receiver, ATI 5000 series HDMI card, and Blu-ray reader or writer.
It's not too important, but also explore the possibility of a HTPC case that looks more like an audio component so that it could fit right into your audio rack.