can i have a killer 2 channel AND HT setup at once

Your C-39 (assuming this wasn't a typo for the C-38 2 channel preamp) allows this, albeit in prologic surround only. The MX-132 (preowned for about $3,300) decodes DD and DTS, while the MX-134 (new, $7,000) allows 6.1/7.1 processing.

You most certainly can use what you have. If you like the sound of the C-39 and MC-352, here is what I would do.
Sell the preamp, and get the MX-132. Probably about a $2,500 upgrade. (a friend of mine is selling his 132 if you are interested) Keep the MC-352 for right and left, and pick up a McIntosh MC-7205 (5 channel amp with three meters across the front) for your center, surrounds, and keep the extra side channels someday, 7.1. The MC-7205 will sell for about $2,200 to $2,500 preowned. I used to use this exact set up, and it is wonderful! Oh yeah, and it looks VERY nice as well!!

Let me know if you have any other questions, I love McIntosh gear!

Dan

Another choice is to get a 5/6 channel power amp that is high enough quality and build to both offer dynamic range for movie soundtracks, as well as finesse for music playback.
This is a tough niche for an amp to fill, but there are a few. Most processors out now will have a 2 channel pass through, that will bypass the DSP chip, and let you use the processor as a 2 channel preamp for stereo playback. This is how my system is set up, with a Cinepro 3K6SE GOLD powering 4 Silverline Audio Sonatina II's and a Center Stage. I use an Aragon Stage 1 pre/pro which has an incredible sounding preamp section. I can switch between 2 channel direct analog stereo, and digital 5.1 with a push on the remote.

Placing any speaker, virtuall, "in the corners" is not going to do well sonically for the most part. Also, the soundstage(if you listen to them on the long wall, spread to the sidewalls), assuming the long wall set up for both movies and HT in the same system, is not going to immage well. Speaker placement and seating placement are both critical for great sound as a foundation.
You will also NEED A DD/DTS PRE/PRO! You can't expect great movie sound without enlisting the help of an OUTBOARD dd/dts pre/pro...simple as that. If you're considering using the analog out's from your DVD to your 2 channel stereo preamp for movies, or even to multi amps, you can forget it..just incase you were considering this.
Also, Are you sure the Plasma is going to give you enough cinematic drama for that room? 42" is small. I've found that a larger picture is a critical element in making the HT experience enveloping! You can easily spend not so much on any number of truely fantastic DLP projectors for this task, and can(even with modest 480P immages) easily get a 70" immage or larger that will look fantastic!
One thought might be to use the plasma over the fireplace for TV watching, but have a screen that comes down in front of it, with projector set up in the rear for movies!...MUCH MORE DRAMATIC AND EXCITING for movies!(once you go large screen/high quality pic, you'll never go back!)
Also, remember that the Grand Piano speakers are better, more laid back music speakers, rather than high dynamic, focused and coherent HT speakers! The pressence and involvment for movies will be more polite and laid back for an HT pressentation...so just be forewarned. You will do much better, indeed, if you do sit closer proximity wise to the speakers. So, in your case, if you listen to the speaker across the short run(speakers and chairs along the long wall, not short wall). If you place the speakers on the long wall, you'll be hearing much more dirrect sound rather than reflected. This is definitly likely if you have your speakers set up more out in the room.
But, it's all trade-offs in the end. If you are more into what you get from the SF's for music, then those might just be the ticket for you. If you wish to Maximize HT, you could try some more "movie" oriented monitors.
good luck

Once you listen to a real high end HT you will mostly listen through a 6;1 system.I would strongly recommend a Meridian 861 V3 processor,a little expensive but worth it.
Ramy