Most any good plastic boot will do, so long as it's a notch up from a base-level model. For sheer bang-for-the-buck, it's almost impossible to beat the Scarpa Inverno ($290), an excellent boot for all but the most extreme cold conditions. And in fact, it can even handle super-cold climbs if you add an Alveolite liner ($150). For Aconcagua, you'll be fine with the Inverno in the stock configuration. You might also want to take a full over-gaiter, such as Outdoor Research's Brooks Ranger Overboots ($125), which also make excellent around-camp footwear. Good socks will be essential, of coursewarm, heavy ones such as Smartwool Expedition Socks ($17) and some fast-drying liner socks.
Other boots will work fine, so let fit be the main thing that guides you. Try the Invernos, but also slip on a pair of Koflach Verticals ($340) and Asolo's Ottomilla ($390).
Have a great climb!