There's nothing bad about resurrecting old threads! There is a wealth of useful information on this forum (and on any forum, for that matter) and it's very refreshing to see people actually use the search option first and then continue an old thread instead of asking a question that many people may already have asked before
If church is the only place where you need more volume than your 20 watt practice amp can produce, I'd say a DI would suit your needs just fine. And it's probably the most affordable option too. Apparently your church has an ample PA and there's on-stage monitoring so you can hear what you're doing.
Many amplifier manufacturers build dedicated bass DI's with a built-in preamp with plenty of tone tailoring options that you can run either on a 9V battery or an adapter. Some of them are pretty affordable too. Some nice examples (click for more info):
Hartke VXL Bass Attack Eden WTDIMXR M80 Bass DI+Tech 21 Bass Driver DI (also available as programmable multi-channel versions)
Seymour Duncan SFX-06 Paranormal DI (no input gain or volume control though...)
Tonebone BassbonePhil Jones Bass BuddyEBS MicroBass IIAnd there are many more. All these DI's behave like a good amp, but in a nice and compact casing that you can just carry along in the front pocket of your gigbag.
I have owned the Hartke and currently still own an EBS MicroBass II. I really liked the Hartke, it is very clean but is also capable of producing a decent tubelike tone with a little dirt and it offers great value for money. It's really affordable, but packed with useful features. I upgraded to the EBS only because I loved my HD350 so much but couldn't always bring my halfstack along.