The HTPC does not need a powerful video card to work fine , you just need one that is capable of streaming video. So the choice to put the 6850 into the HTPC would to be solely for the fact you want a highend card in your gaming machine and you just want to get use out of the 6850.
MSI R7950-2PMD3GD5/OC Radeon HD 7950 3GB 384-bit GDDR5 PCI Express 3.0 x16 HDCP Ready CrossFireX Support Video Card
$399.99 and a $20 rebate makes the final price $379.99http://www.newegg.com/Product/Product.aspx?Item=N82E16814127651
With a budget of $300 to $400 you could get this card and make your gaming rig much more powerful.
Of course the less expensive way out would be to get another 6850 and put the 210 in the HTPC.