As for what video cards, nVidia is really the only choose since the motherboard is an nForce SLI board. As for which video card you want, that depends on how much you want to spend. But remember that just because you have the option for two video cards doesn't mean you have to put two video cards into the system. You could put a nVidia 7800 GTX into the system, wait a year or two and then put in a second 7800 GTX when the price comes down.
I don't really need SLI as the most video intensive things I do are SFC, Never Winter Nights and play DVDs. I'm not into things like Doom 3. I probably won't put a really expensive card in as I hadn't really planned on building a new machine anytime soon, but if I get the free CPU it is just
too tempting to resist.
What I need is a card that can handle those jobs and run at 2048x1536 with at least a 85 hz refresh rate.
I'll likely eventually add a 2nd card and go dual monitor though.
I may look into
MythTV for the Television side of things. It ought to be a learning experience but that is half the fun at least. Setting things up under Windows would be much easier (due to my Windows vs Linux knowledge levels).
As for which type of OS to put on the system, you could always go duel-boot route. Or for that matter you could use your current system as a Linux file server for all your movies and other data. That would free up your new system for Win2000/XP gaming machine. Since I'm not really sure what you have in mind I'm not really sure what to suggest.
Dual boot for me is an issue as I won't buy Windows XP (call it activation phobia) and the older Win2k is hard to find. Either the new machine (if built) will be Linux or the older machine will be converted to Linux and the new machine will get the old Win2kPro.
The "old" machine is quite powerful in its own right. Athlon XP3200+, ASUS A7N-E Deluxe motherboard, 1gb ram, 250gb HD, ATI Radeon AIW 9800Pro, DVD+/-RW and CD R/W.
Then of course there are the two dual cpu machines... Which of course explains how I got to be #4 on the SETI team (onginal not BOINC).