No such thing. Even if you could mod your own modem, the one you're dialling into at your ISP will still be a plain old 56k modem.
At best you might be thinking of a way to modify it so that the communication from the modem to your computer can be increased. A standard serial port operates at 115Kbps maximum, and most modem drivers configure themselves to use that speed for the computer to modem communication. A later version of the standard port, which is highly uncommon, allowed 230Kbps. If your modem's chip supported it, you could then provide more data to the modem. However the modem is still limited to the 33.6Kbps (or 48Kbps for V.92) for the actual data transfer.
Most modems do some sort of compression of the data. If the data is text, then a significant compression is possible, so that perhaps 50k of text results in only 5k of data transfer, with the modem at the other end decompressing it. In this case, a faster serial port speed results in the modem getting more data faster to be able to do better compression. However, most of a modem users upload is just connection requests to servers, or IRC/AIM/ICQ chat, which is just text data, and very little of it. The compression doesn't really result in very much difference in speed, and the inherent latency of a dialup line makes it pretty much unnoticeable. On the download side, whether or not extreme compression occurs depends on your ISP's modem's abilities; and given the content of many webpages, the amount of compressible text isn't much these days, while the uncompressible images or other objects are becoming greater.
Also, the limits of analog modem use are a result of the physical limits of analog phone lines and the switching systems. The FCC limits what is allowed on the phone lines, so that the limit for the download is now 53Kbps for a 56k modem, and the only way that is possible is if your phone line is digitally transferred through from your ISP through the phone network all the way up to the last point where it goes analog to your phone. The upload limit can't even go that high, because the signal has to start out as an analog signal which can't be gotten up to those speeds. V.92 modems apparently don't really get up to 48Kbps uploads, they're just able to do it in the right conditions.
Also, external modems DO have some differences from many internal modems. Many of the cheaper internal modems are "software modems" -- the chip onboard doesn't do any of the signal processing, it just converts the signal to a form to send to the CPU, which then converts it to digital information (some types do a limited amount of processing so that it's not entirely software processed). A modem which does the entire process has a Digital Signal Processor (DSP) onboard, which converts the analog signal directly to the original digital data stream that the original computer was sending, then sends it to the processor for storage, display, whatever. All or at least very nearly all serial-port modems are hardware modems, simply because of the low data rate compared to a PCI bus, and external modems are already expensive anyway so you may as well be getting the best type possible. USB modems I think for the most part are software modems, but there may be hardware-based versions.
A software modem uses CPU cycles for the modem processing. This means that the other stuff you're doing doesn't have as many cycles available. On a very slow CPU, this can mean a significant portion of the CPU power is being used just for the modem transfer, and in the past there've been issues with software modems not always getting as good a speed as a hardware modem, or the noticeable speed seems slow because the processor isn't able to deal with the data fast enough. However, with today's CPUs, and very well-tuned software modem chips, this isn't much of an issue. Some people get better performance with a software modem than a hardware modem.