Good equipment is a must. For footwear, take a pair of light hiking boots for the hike up to base camp. For the climb itself you'll need a pair of expedition-type plastic boots. Scarpa's Inverno boots ($300; www.scarpa-us.com) are an excellent choice that won't break the budgetthey're warm, reasonably comfortable, and perfectly capable of handling the climbing on even the more-difficult Polish Route should you take that instead of the standard route. You can buy an extra-warm liner for another $198, but I really don't think you'll need it. Fit the boots at least a half-size larger as your feet will swell at altitude, and take good-quality boot liners such as Wigwam Gobi Polypro Liners ($4.75; www.wigwam.com) and warm socks such as SmartWool Expedition Trekking Socks ($18; www.smartwool.com).
Other boots that would work well include Asolo's AFS 8000 ($400; www.asolo.com) and the Koflach Arctis Expe ($365; www.koflachusa.com). Both are warm, rugged boots that can handle cold temps and crampons with ease.
By "parka," I'll assume you mean a cold-weather jacket. Clearly, a good-quality down parka is in order. Alas, this will not be cheap. Marmot's estimable 8000M Parka, for instance, sells for $575 (www.marmot.com). It's made with super high-quality down and has a Gore DryLoft shell to repel moisture. More affordable is Feathered Friends' Frontpoint Jacket, which is not technically an "expedition" parka but would work well on Aconcagua. It's $385 with the eVENT shell (www.featheredfriends.com), an excellent shell material for this kind of jacket that's very waterproof yet highly breathable. If you tend to get cold, Feathered Friends's Icefall Parka ($435 with eVENT) is a little longer and has more fill, promising real go-anywhere warmth. You'll probably never use it again for the rest of your life, but you could sell it on eBay (as I write this one is selling there for $248...).
So those are my recommendations. Good luck with the climb! To me, Aconcagua looks like a big, cold, dusty heap of ice, but that's just me.
Read up on more expedition parkas in Outside's "Cold Play" (November '04), plus check out our guide to climbing Aconcagua.