I need to transcode MPCs to MP3s to use on my portable player. Is there a program out there that will decode the MPCs using the ReplayGain tags, then transcode to MP3 & copy the tags without going to WAV first? I am familiar with Speek's frontends and I don't think I can do it all in one go from there. What about the other MPC/MP3 frontends at Rarewares?

I could simply transcode to WAV using ReplayGain, then encode to MP3, but then I wouldn't have the tags, and I'd have to format the filenames, I could also transcode directly, retaining the tags, but then I wouldn't have the ReplayGain info, and I'd have to use MP3Gain, blah, blah, blah ...

Any suggestions?

Foobar2000's diskwriter (Convert from right-click) can have Replaygain enabled (and it can even use DSPs, such as crossfeed if you're going to listen to your MP3s on headphones and like the crossfeed sound). Case's Foobar2000 installer including 3rd party plugins is at:
http://www.saunalahti.fi/~cse/foobar2000

Using Case's foo_clienc commandline encoder interface available from the same folder, you can convert to MP3 using your favourite encoder, such as the Lame (ID3v1/2 tagged) preset for clienc (which has --alt-preset standard -Z as the commandline). CLI encoder uses "standard input", not an intermediate WAV file, to send the data, so you need '-' as the input file (included in the presets).

You can even preserve maximum quality (outweighed by the loss of transcoding, though) by piping 24-bit output to LAME. The commandline option --bitwidth 24 will do that, though CLIENC or LAME might deal with that without the switch (I'm not sure).

If you want presets like --preset medium you can use an encoder that supports it (e.g. lame-3.93.1 or search the forums for the new lame-3.90.3modified.zip) and modify clienc's commandline box to that commandline (without deleting the '-' that specifies standard input).