My goals / thoughts, in order of most important to least important:

1) A Tier 1 Frontend + Backend system for SDTV
2) An almost entirely pre-assembled system, except I'm willing / able to install video cards and hard drives
3) I think I want two Hauppauge 150s (the single 500 sounds not-as-good)
4) I think I want a non SATA to boot from, and a SATA 500 GB to store on (unless you guys have a better idea for getting to 500 GB cheaply and reliably)
5) I think I want wireless 802.11 g to get on the internet, as an RJ-45 to my living room is impractical
6) I'd want a DVD RW
7) It'd be awesome if I could later get one or two MediaMVPs running as frontends

I'm a hardware *moron*. I will pay more, as long as it decreases the likelihood of me having to crack the box open again, after I install the cards and drive(s). I'm also not super keen on spending time fiddling with settings and scripts... I'll pay more to have less editing to have to do...

My thoughts on "cheap" for the base system are in the ballpark of $400-$500 USD, if possible. I figure I'll be spending about $160 on the 500 GB hard drive, and $150 on the two 150s, so my total system cost should be ballpark $700-$800, right? Or can I do better?

Maybe a Dell E521n with Athlon 64 3200+? $481 USD.

Any better suggestions?

I think you and I come from similar places. I ended up building my system. Its not that hard really. Mine is

Antec NSK2400 case - to me its silent, and looks like it belongs in my entertainment center

SATA 250 hd only - yes I need more space, but you can boot from SATA just fine. FWIW I haven't read that SATA works any better for myth than IDE, so maybe just go IDE. I also spent about $80 for 1gb of ram, I thnk I could have gotten away with about half that. And every other week someone has the Memorex 18x DVD writer on sale for $50. Graphics card could have been a 5200, these are under $50 at Compusa, I paid a little more for a 6200.

$150 mobo/CPU combo from Compusa (now $100 at the GOB by me). On the net you could probably also do this combo for about $100. Mine is a Sempron 2400+. I do dual tuner OTA HD with NO problems or issues. You don't need a faster CPU unless you're going to skip the graphics card or do a LOT of archiving/transcoding.

Everything I bought was store bought except the tuners. Cost was $650 after rebates, and buying on the net would have brought the price down to under $600.

It was all easy to put together. Store bought boxes can be had for $300 if you're patient and look hard enough and will wait for a rebate. But they're loud and ugly in the living room and won't even fit in a TV cabinet. Myth requires getting down and dirty with the software to get it working right, and that was much, much, much harder than screwing a handful of components in place. Good Luck!