Here isn't the place for gaming, at least not commercial games of the dreamcast. Subjects will be hardware mods and dreamcast programming.

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To build dreamcast extensions in place of the modem, you'll need a connector. Those connectors doesn't exists on the market, here's How to desolder your useless modem connector.

When you've got the connector, you'll need a printed circuit board on which to solder it. T. Komoriya made a pcb for such a circuit, but I wanted to be able to mount it in the original modem box, I hacked the original pcb and here's the result (I just moved the 50 pins connector and reduced the board).

Some pictures of the realized pcb here.

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Maybe you've heard of fast (1,56mbps) serial cards for the dreamcast, here's some work on making one.

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Here's my own dreamcast cdrw tutorial, much easier than those soldering wires directly onto the optical unit's trimmer.

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Some software now :
Here will you find testvmu and restorevmu.
The testvmu program is intended to test vmu's for bad memory blocks, it performs a global read and saves it for later restoring, then writes it with four types of bytes and verifies it, at last it writes back the original state.

If something goes wrong (I tested it, if the vmu is not restored the dreamcast won't see it anymore) you should be able to restore it to a blank state with the restorevmu program, restorevmu could also be used to save otherwise seemingly dead vmus.

I only tested this on official 200 blocks vmu, it should be easy to modify it for bigger vmus (see the defines at the top)

This one's called md5.
md5 is a brute port of a source I found on the net, it's goal is to verify if serial transmission with the dreamcast is error free.

The virtual disk contains a 4 Mbyts file, uncompressible with the standard build of dc-tool. The program starts with a few standard md5 tests and then verifies if the testfile was received without any error by calculating it's md5 signature.

These programs come WITHOUT ANY WARRANTY, in the hope it will be useful.