To determine whether you need an alternate or not:
1 hour before, 1 hour after, 2000 foot ceilings, 3 SM vis. --- anything below that, you need one.

Airport w/ a percision appch: 600 foot ceilings, 2 SM vis.

Airport w/ a non-percision appch: 800 foot ceilings, 2 SM vis.

Landing VFR --- you must be able to decend, and maintain VFR throughout the entire approach-to-landing.

Hope this helps...
Lax