integrated sound is fine for htpc's imo, you can even get away with integrated gfx as well. however a separate gfx card is not expensive to do especially the 8600gt series.
psu - i agree it is probably best to get a new psu, they can be had a reasonable prices, especially for htpc where you don't need a massive psu e.g. this corsair one Corsair VX450WUK 450W PSU - Ebuyer