To further complicate matters, you may want some solar gain during the cold months, which means double panes are better than triples. In my part of the country the conventional wisdom is that double pane makes most sense. During the winter you want the drapes down at night for extra insulation, open during the day to allow solar gain. During the summer you do the oposite.
Be careful buying replacement windows. There is a big variety in price and performance and there is not necessarily a coorelation between the two. I woud stay away from aluminum for various reasons. Wood framed windows with a vinyl cladding is generall considered the best but there are some very nice all vinyl windows out there. Shoot me a PM if you're interested in comparing brands. I know most of them, but there are so many mom and pop window manufacturers that I can't keep track of them all.
Remember to compare apples to apples. You need a remodeling product, which is why Marvin's custom sized vinyl clad wood windows is a good option. Their larger and better known Minnesota-based competitor is one of my clients, so I'm biased toward them, but anyone who makes windows to fit your rough openings should be considered.
Since you already have some vinyl windows in the home, matching appearance is probably more important than performance. That means you'll want to stay with vinyl windows that look pretty much the same, or even the same brand. One of the best value vinyl windows out there are Silver Line
http://www.silverlinewindow.com/index_noflash.cfm As a disclaimer, I do represent Silver Line and their parent company, so take what I say for what it's worth, but do compare Silver Line when you're looking.
Edit, to answer your second question, casement windows are generally considered the best performing style, although any style should work just fine. Casements have the fewest moving parts and are an inherently stronger design. The sills are sloped so that water flows out of the window so the bottom of the casement isn't sitting in water. I would match the appearance of the other windows first but if style is an option, I'd choose casements.