In that price range the Beringer Truths are generally considered best.
Alesis monitors have a tinny grainy sound that is especially harsh in the upper registers, I would definately giving them a good listen before purchasing.
Honestly, if you want a pair of monitors that wont leave you wanting for more in a few months, consider saving a little more anf getting the M-audio BX-8's for $500. In fact there are several quality options available in the $500 range that are quite noticably better than anything in the range you mentioned.
The BX-5's are in the price range you mention, but i would caution against using monitors with 5'' drivers as they will not accurately reproduce the low frequencies that are so critical to a clear mix.
Hope this helps and good luck.