While it ;is usually said that the system is no better than its lowest quality part, you will get a bigger difference in sound based on speakers selected than anything else in the system. Yes you need a decent amp and sound source, but the speakers make a big difference in how things sound. While I do have some brand preference I will not pass that on not knowing if anything they make would be within your budget.
I will however suggest what to do to select a speaker,. Take a CD with you of music you like and are familiar with. Listen to it on high end equipment and speakers. Try a few different ones to get the sound you think is appropriate. Then audition equipment and speakers within your price range. Get as close to the high end as you can within your budget. Musical sound is very subjective and the ideal varies considerably depending on what type of music you listen to,.
Bottom line is I would suggest spending more than you want to on the speakers. They last way longer than anything else in the sound system. If you are concerned about quality of sound as opposed to just noise, it is an investment worthwhile making.