Now replacing speakers is more of a subjective thing.
What i can recommend are a pair of 5.1 speakers from Altec Lansing that were branded by Dell. http://www.directron.com/ada995.html
Are the ones I bought them off of someone on Craigslist for 90$ 2 or 3 years back. Their probably some of the best bang for the buck speakers money can buy quite honestly. I've just been floored by their quality time after time.http://www.newegg.com/Product/Product.aspx?Item=N82E16836121120
Are the route to go if you want speakers new.