How can I beat "Big Money" in Dominion?
There comes a time in every Dominion group's life where they figure out that Big Money (the strategy you describe) is beating everyone else who is going heavy on action cards. Do not fret - it's most often not actually the best strategy.  For example, if everyone else is going Big Money, buying a single Smithy will win a 4-player match about 60% of the time and a 2-player match about 80% of the time (via a trial run of 5000 matches on my simulator program).

There is pretty much always a superior strategy to Big Money on the table; you just have to find it.  A common mistake is to spend turns buying too many Kingdom cards to have giant crazy combos that don't pan out. Another thing to watch out for is going for a longer game with a fantastically combo-heavy deck, but then by the time your deck is all situated someone else has bought all the Provinces.  Even still, combo decks do sometimes work - I have been in some thrilling Village+Bridge decks that beat the pants off Big Money.

I haven't been playing around enough with it to know for sure, but it seems that the new expansion Prosperity also lends itself toward Kingdom card strategies winning more often. The reason I feel this way is because Prosperity seems to stretch out the game a big longer such that bigger decks with lots of cool combos can succeed quite well.

Generally I like to measure my strategies against Big Money - if it can't reliably beat Big Money, it's not a good strategy.

But even still: when in serious doubt, buy a Silver.