Arts groups may have a tough time prying much money out of the government these days, but 2004 has been a record year for private giving, with enormous single gifts dominating the philanthropic landscape. The biggest donors of the year were Bill & Melinda Gates, who pumped a whopping $3 billion into their own foundation, and while most of the truly outsized gifts went to universities and foundations, arts groups got their share of the largesse as well.