Extended battery uses same cover.
Motorola OEM Droid X / MB810 Extended Battery BH6X
It almost fits better in the battery-well than the standard battery. The battery is a bit thicker 1-2mm and is flush while the stock battery is below the surface a bit. The battery door slides back on no problem and is perfectly secure.
The stock battery BH5X isn't a slouch but only has 1500/1540 mAh,
the larger BH6X has 1880/1930 mAh, a 21-22% increase.
In practice that is about right. With the old battery after 10 hours I useally had 20-30% after last night I had 50%.
You can get one at Amazon for $14 for the battery and $8 for the charger.
Now I always have a charged battery at work, so I can used the Hell out of my phone. Sometimes I try to see if I can use up the battery. It's the advantage of an exchangeable battery, if it gets low, pop in a fully charged one and go. I do a lot of hiking and bike riding with GPS constantly on, having an extra battery along is very comforting.
Now I'll have to get another charger so I can keep a fully charged battery at home too.
Update: Well I did it, I got another charger and extended battery, so now at home and at work I have a fully charged battery ready to go. So if I ever forget to charge my phone (or leave it in my car over night) when I have to go 'now' I've got instant power.