Just to reiterate what has been suggested above, Diablo 3 isn't likely to be too demanding so your current system will likely cope quite we'll. The 5850 is still a good graphics card and holds it's own when compared to new £100 cards. I would probably throw on an aftermarket cooler and try overclocking your processor a wee bit if you haven't done so already.
As for a media system, there are quite a few options.
1. AMD Zacate, ultra low power, integrated basic Radeon gpu on the CPU. Plays 1080p video fine but I've found if streaming, playing videos & downloading simultaneously it can get sluggish. I've a totally.passive board so is very quiet.
2. Basic Intel Pentium/Celeron with passive Radeon 6450.
3. AMD Llano. Sweet spot is probably the tri core processors and will cope with a bit of low level gaming.
As for the SSD, I love mine in my htpc but could live without the one in the gaming rig.