Here's part 2 of my summary of different categories of gear I used on Aconcagua. Part 1 is here.
For the trek in, you definitely need good sun protection, and a broad brimmed hat is preferable. I wore my trusty baseball cap with a buf arranged in Foreign Legion style to protect my neck. It wasn't quite enough. Two members of the group had the integrated floppy hat and neck flap made by Outdoor Research. These hats are far from stylish, inspiring both good natured ribbing and self-deprecation from their wearers, but seemed to work well. Recommended if you don't mind looking like an eccentric scientist catching bugs.
A warm hat is another essential. As I've described elsewhere, I took two. The North Face beanie later became a beloved item and was almost permanently attached to my head during this past winter in Wellington. However, on the mountain itself, my alpaca super-chullo, hand-produced in the village of Callalli in Peru, was unbeatable for warmth and comfort.
A lot of attention needs to be paid to getting the right mix of hand protection. As I found out on Nevado Ampato, if you can't keep your hands both warm and usable, you become helpless pretty quickly.
The gear list recommended two pairs of liner gloves, fleece gloves, mountaineering gloves, and expedition mittens.
As my basic liner gloves I took Outdoor Research PL 100 fleece gloves. I can't say enough good things about these gloves. They are warmer than you'd think, and are snug and stretchy, allowing good dexterity. After lots of scrabbling around with sharp rocks trying to anchor the tent I eventually destroyed the fingertips, but I made sure I bought another pair as soon as I got back to NZ.
I also took some Outdoor Research PL400 gloves, which are quite a lot thicker. These weren't particularly useful, as they didn't seem much warmer than the PL100s and weren't water or wind proof, but still prevented me from doing much with my hands -- I couldn't even get my fly undone while wearing them. If I had my time over, I would take another pair of PL100s, some windstopper gloves, and/or a thinner pair of fleece or wool liner gloves.
For my mountaineering gloves, I had a pair of Outdoor Research Arete gloves. Mine were an older version than the ones shown in the link and didn't have any insulation. I dispensed with the fairly useless factory liners and wore the outers over the PL100s. These provided adequate protection on the lower mountain when it was snowing, and they have a good idiot cord system. Starting over, I would prefer some gloves with insulation, which would work allow the option ofwearing them with thinner liners.
Finally, for the uber-warm mittens needed on summit day, I took the Black Diamond Mercury mitts. These were plenty warm enough and have an austere yet cuddly feel which makes it a little disappointing that another high-altitude expedition or camping in Siberia would be the only other occasions I can imagine wearing them.
As noted elsewhere, I would also highly recommend having some chemical hand warmers, as no matter how warm your mittens are, there'll be times you'll need to take them off, and they may need some help to warm your hands back up.