Video Card for HTPC with older but higher end mb

Hey all,
I'm putting together a HTPC using some good parts I have available, and looking for some input on the video card to complete it. Or if you think this is just a bad bad idea to begin with, let me know. I know that mini or micro boards are better for these builds, and this is a full ATX board, but it is still pretty high end (supports 16gb memory etc), and I'd rather use it since I have it than spend money I don't have on a new one just because it's smaller, plus a new CPU etc etc.

I have:
ASUS P5Q-E LGA 775 P45 ATX Intel mb newegg link
Rosewill RP550-2 ps newegg link
I have 2 LGA775 processors lying around to choose from:
Intel Core2 Quad-Core Q6600 2.40GHz
Intel P4 631 3.0GHz single core (yes, supports 64 bit and 2MB L2 cache- I know it's old but still works great. Do I need the quad core or the faster single core?)
4GB DDR2 1200 memory
Not putting an optical drive in at this time, I have a separate player for that, although I may in the future.

Need:
Video Card: Was eyeballing ASUS EAH5450 because they're cheap, but is fanless a bad idea? newegg link I don't need low profile. I'm wondering if I need a NVidia GeForce card.
I do have a couple Radeon HD4850 512M GDDR3 cards laying around, very good cards and capable of Crossfire together, but they are dual DVI, no HDMI. Is it worth trying to utilize them with a DVI-HDMI cord?

Also need a case. Size not really a concern, I have a couple laying around but might splurge on a new one if there's any decent ones that will fit ATX board with PCI video card.

This HTPC will be used exclusively for streaming videos via network and for light gaming (16 bit emulators ala xsnes9x, no big gaming). Will probably run XBMCbuntu or XBMC on Win7 x64, or maybe both as dual boot to see which I like better.
I do not need 5.1 or 7.1 audio, I'm only running 2 channel stereo. Looking for HDMI capability, don't even need audio through HDMI right now. All set with HDD, although I do need to get a remote, but that's easy too.

Thanks for checking out my post, and I look forward to your opinion!