Unless you're staying at stratospherically luxurious hotels, you'll find that people wear a range of gear; and having travelled fairly extensively in the tropics I would always recommend travelling light, and dressing for comfort. Natural fabrics, eg cotton and linen, are best, The main stem - ie the main pathway leading directly to the Taj - certainly gets crowded with tourists - ie people just like you, me and cool breeze! - but if you stray off into the gardens to the side you will find it much quieter, and you may even get a spot to yourself, from which to view this unique monument, before (or after) you actually go into it. Cool breeze is a little off the mark, or joking, with his "wee train ride" between Varanasi and Agra. It's about 300 miles, which in India could take about 10 hours, plus waiting time - we once waited about 7 hours for a train from Indore.