I use Outlook express with SP1 and several email accounts (hotmail, tweaktown, my ISP's, etc.) and it checks them all rapidly and without any problems. Not sure why this is happening to you Darth.

As for "SP1 Tweaks" well I don't think there are any tweaks specific to SP1. Try my XP Tweak guide (sticky at top) and you'll basically have all the known performance tweaks available for XP with or without SP1.