AAWP have scoured Marketplace to find a full list of Windows Phone apps which will not install on 256 MB Tango handsets.
They found 1,387 apps were barred; only 1.6% of Marketplace. Unfortunately these apps were clustered towards the high end of the market, with a full 25% of Xbox Live apps disallowed, and 5% of the top 10,000 apps by downloads.
AAWP notes however that very few new apps are 256 MB incompatible, just 0.8% of new Windows Phone 7.1.1 SDK apps do not support Tango, and presumably as these apps are updated the numbers will drop even more.
Even then, buyers should definitely know that it is worth spending a bit more if they intend to game seriously on their Windows Phone.
AAWP has a full list of all the 1387 apps, which can be found in their detailed post here.