I'm not happy with my pvr-500. I'd go with two pvr-150s instead. My reception is very poor on the 500 compared to the 150s and I have very good rf signals coming in that are in good power ranges. You may get better results but for me the 500 is only good for svideo capture of my cableboxes
that be-2400 chip will have problems with x264 and h264 encoded hd material. It should be able to handle 1080i and 720p material ok. If you plan on using XvMC I'd advise against it as it has never worked right for me since myth version .18. I went with a intel E2180 and overclocked it to 3.1 ghz easily without boosting cpu v-core (voltage)
for an idea of the hardware I'm using here you go: http://mythtv.org/wiki/index.php/User:Blackoper
I'm not sure about that Caviar disk array. I remember hearing their drives are crap. You may be better off buying two $200 western digital 1tb drives and either putting them in e-sata external cases or make your own array case on the cheap. You will probably be much better off and have better speeds and longer drive life.
You are paying way too much for your ram. You should be looking for high performance ocz or crucial ram for $30 to 40 after rebates and a latency of 4. Check out deal sites like fatwallet.com or slickdeals.net and their hot deal forums to save some money on your parts.