I just downloaded and installed the pmdg 737 a few weeks ago. Did that already come with sp1? Or do I have to update it? Also do I have to uninstall it first? Then do a fresh install and update right to sp1 before anything?

All ready has been posted before  to your question no since you have just downloaded  the ngx  it will be up to date to the latest sp

So since i just

Downloaded it, i should be all

Set

Then?

The download has been fully updated. Even the older NGX boxes are up to date.