You can check out prices at places like MacConnection or MacMall. The less expensive Macs are probably the iMac (starting around $1000 - it's an all-in-one unit with built-in monitor and comes with mouse and keyboard) and the Mac Mini, which is just the computer CPU and needs you to add monitor, keyboard and mouse.

If your current keyboard is a fairly recent USB type, it will probably work. Same for the printer (though you'll probably need to download the Mac driver for it). If your monitor is fairly recent and has a VGA connector, it should work. (So I'm probably saying the gear from your 2-year old eMachine will probably work, but the the gear from P-Vine's ancient beast probably won't - though it might work on your eMachine - don't know what sorts of connectors they have.)

I am curious - I like Macs, but they are different than PCs, and there are a number of fairly affordable PCs out there - are you sure you want to switch?

All the best,
JudyB