Vegas is DX9 based unlike most others that are OGL, and it's not vendor specific unlike many other NLE products that prefer one architecture over the other, and also usually lock you in to a workstation card to enjoy those benefits.
Vegas will like a good DX9 card when working with the max preview window setting.
AMD recently added an acceleration plug-in for Vegas, but I haven't tried it yet;
http://forums.amd.com/forum/messageview.cfm?catid=376&threadid=118647
It was tested on a Phenom & HD4K but should not be limited to that.
If your budget is $300-400 for the graphics card, then I would recommend the HD5870 or HD5850, which gives you triple monitor support out of the box and has a nice compliment of connections. Probably the HD5850 would be fine, and then you can redirect the savings into other parts, like more memory or more HD space;
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=2010380048&QksAutoSuggestion=&ShowDeactivatedMark=False&Configurator=&Subcategory=48&description=&Ntk=&CFG=&SpeTabStoreType=&srchInDesc=5850
Or for heat & power reasons if you can wait, then an HD5750, hopefully with a quiet HSF. The new added math in the ALUs (+SAD) might help future encoding and decoding if they build a good plug-in to take advantage of it.
If you want to save some money then I think a nice HD4770/4750 would be a nice fit, and you can upgrade to a cool & quiet HD57xx series or nVidia equivalent (if there is to be one) next year;
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127452
There's also the fanless HD4850 too;
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125258
but I would prefer and HD4770 for better heat and power. Although if you dont' care about those things, then the HD5870 is the best and give you 3 monitor support out of the box which is handy for editing.
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