Buy a good soundcard first (preferrably external).
Then I'd recommend a set of powered studio monitors (events/tannoys/jbl/etc). Add a sub or two as needed.
Like robb_lowe said, the KRK line is what u should look at.
Either the RP5's or RP6's.
And Symphonic I have never used the Alesis, but u should see if u can go in store and just listen to em.
Thats the best thing 2 do.
I have a pair of Alesis Monitor One's they are ok, the bass response is a bit inaccurate but I have mixed many albums on them. go to my site and listen to the fourth time around songs, they where mixed on the Monitors ones
ok, i don't have monitors so i can't realy hear the diffrence i think i'm used to hifi speakers and headphones so monitors will all sound good at the begining. i'm mixing with headphones and that sucks on the low freq.