I've got my frontend setup almost exactly like yours, but I have an nvidia card and use mythtv and xbmc as my frontends.
I don't have any tuner cards or anything in my frontend, that's all in my recording backend.
The difference between spinning disc and ssd was amazing. After POST, I've got my frontend boot time down to 11 seconds. This means booted into mythtv, ready to accept remote input. I don't bother with sleep mode any more.
Even if you don't have hw acceleration in the radeon, the core2 is plenty enough to play back streams provided you use a multithreaded decoder.
I actually have the xbmc database locally on the ssd. It doesn't seem to have any adverse effects.
2 GB RAM
64GB Kingston SSDNOW!
Asus P5Q board
nvidia 9500GT?? I can't remember now...
I have had no issues with this setup, even with playing back stuff encoded with 10 bit h264 profiles.
My server is a little more beefy:
Q6600 (3 threads for commercial flagging
8 1TB Seagates in RAID5 using rocketraid 2320
1 300G hard drive for OS & scratch & databases
1 HDHomerun DUAL
I've discovered that my raid hiccups when recording 3 or more shows (I should've spent the extra $150 on the raid card that had a dedicated processor! damn), so I'll be eventually getting a dedicated 2 TB for mythtv recordings.