Thanks to price wars, most cpu/MB combination's available are plenty fast enough for HD video (be aware, if they are dvd rips, the are SD video, and even if they are output at higher resolution, there is a lot less source information, so they take even less, substantially less power to play). Dual core athlons have become entry level (it wasn't that long ago they were top of the line). If you think you may ever change your mind about converting to h.264 or anything like that, a more powerful processor might be helpful. As far as video playback, its plenty, and as far as ripping, ripping doesn't take a lot of cpu power. My htpc is running on a single core athlon 64 and plays dvd rips just fine (SD doesn't take a lot of power). I would personally get a dual or quad core intel if I was building a new system, but its not necessary if you want to keep cost down. About a MB with onboard video though. The links not working for me, bet I assume its microatx. How many pci slots do you have (enough for a sound card, tuner card, sata controller and anything else you might want to add?
If the onboard video doesn't prove to be enough, you can always add a video card. Ati seems to be what everyone prefers for htpc's (including me) so look at 2***/3***/4*** series cards. You don't have to get a 180$ card either (unless you really want to spend that much). Video playback (even HD) doesn't require high end video cards like gaming does. A 50$ ati card will do the job quite nicely. The onboard will most likely play most if not all Hd video, and will play all SD video, and you dvd rips ar SD video. The option to upgrade and ad a card still remains.
I would NOT go with that antec power supply though. Antecs are good (I'm running antec power supplies in all my computers because you can get them on sale cheap), but its NOT big enough. I would have though that with all those drives, it wouldn't have been big enough, but I have an lg gh22ns30 in front of me and crapped my pants when I looked at the power requirements. Most opticals take about 1.5 amps give or take but this one wants 3.6 amps!!!! That's 2 amps on the 12v line, and 1.6 amps on the 5v line. Lets look at your 12v requirements alone (I'm assuming the s15 is similar in power to the s30).
6 lg opticals 12 amps (144 watts)
6 hard drives 6 amps/72 watts (guesstimate, couldn't find specs on the wd)
65 watt x2 (realize some other processors may take 100 watts or more).
add in video card if you get one (70 watts or less if it uses the pci-e slot only, more than 70 watts if it has an extra power connector). Glancing at pictures on newegg, several 4830's have an extra power connector (starts at about 90$) while the lower models don't seem to. Lets figure 70 watts.
That alone is 351 watts and that antec power supply is rated at 360 watts max. Several other parts use 12v power, so its not big enough. Lets add the 1.6 amps each lg takes on the 5 volt and 0.5 amps for the hard drives (63 watts total), and you are now at 414 watts on a 430 watt power supply. We haven't added power for memory, sound card, controller card, fans, motherboard, usb and fire wire devices and everything else. If you want that many drives, you need to go with a pretty big power supply. Antecs are fine but you need a bigger one (and if you want an antec, watch for sales on newegg, frys and micro center).
There is no reason you couldn't go with 4 1.5 TB hard drives or 3 2TB hard drives either.
As far as hooking it up to the tv(s) and stereo, you mentioned ps3 etc. Be aware for your main tv, your video card can output straight to it (many cards can adapt to component, composite, s video, hdmi or whatever you want). Your sound card can output direct to your sound system. You can run an htpc front end (software) so that you can see, be able to read, and control media playback, tv viewing and recording and such from across the room on any tv, and you can use a remote control to control it.
As far as other rooms, 10/100 Ethernet does sd fine and 10/100/1000 does HD fine (you might even be able to do some HD on 10/100). Are there computers in other rooms where there are tv's? All you need to do is make the hard drives viewable to the other computers (share them and or remap them). You can basically just play everything directly from this computer you are looking at building on your main tv.