I'm doing runs of cards on my 3880, which is probably the best card printer on the market (only one I can think of that supports small sizes in the sheet feeder, yet still has big ink tanks, reducing running costs to 1/2 of what you'd pay for a smaller printer). I realize this doesn't help people who already own the 4900, but if you're looking for a printer and cards are part of your output, the 3880 deserves a very serious look, because it is the only inkjet around that really handles them in production quantities. Note that the 3880 is a pain at the other extreme end of its output range - it prints 16x24, but only by cutting paper down from rolls because the standard paper size is 17x22, rather than 17x25 and the 3880 lacks a roll feeder. Probably the ideal combination of printers for a small studio is a 3880 for cards through 8x12 or 12x18 prints and a 7900 or 9900 for large prints - an iPF would also work, but the two Epson stablemates keep the Epson look - this is expensive, and I don't have the big printer - I print 16x24 on the 3880 by cutting paper down, and I send the big stuff out. If I get the volume to make two printers worthwhile or get into a situation where I can share a 9900 with another photographer or two, that's where I'll go.
I run Moab Entrada natural prescored stock in the sheet feeder, using the settings I'd use to make a regular print on the same paper (Moab's icc profile, print settings for Epson Ultra Premium Matte (Moab's recommendation), 1440 dpi). At these settings, the output time is about 1 minute per card, and the sheet feeder holds between 25 and 50 cards at a time. I'm generally sitting next to the printer doing something else on the computer (I'm running 250 cards in the background while posting on LuLa right now), in order to keep an eye on the process, but I am pretty sure I could also leave it completely unattended and go for a walk). I buy the Moab paper in 250 sheet boxes, for about 60 cents per card.