Without going into an incredibly long lecture (you can always go to http://www.coffeegeek.com/
for more info that you could ever need!), probably the best investment is a quality grinder (a burr grinder rather than the whirly-bird chopper type) - with this and freshly roasted beans you can make good coffee in a variety of ways. Sunbeam now make some grinders that can work for grinding from coarsely (eg for plunger) to fine (for espresso machine) - the Sunbeam EM0450 is under $150 and the EM0480 (slightly more robust) is under $200. You can spend a lot more than this, but these are a good start (think Tiagra and 105!
Whatever method you want to use to brew your coffee, a decent grinder means you can grind fresh when you need it, which is the key to good coffee.
I currently use a plunger at work and an espresso machine at home. If you think the plunger coffee is too weak, use more coffee (though I think more than 2 teaspoons of FRESHLY GROUND coffee is enough for a large cup / mug).