Receiver vs. processer?

The answer is completely dependant on whether 2-channel performance will be an important factor for you. If you are used to good 2-channel performance and want to maintain two channel but also have surround for movies, then keep your preamp and power amp and add a pre/pro and 3-channel amp. If you are more flexible and less demanding for 2-channel, get a pre/pro and amp(s) or a receiver. They are cheap, easily replaceable and not very good for 2-channel performance.

I have owned some of the nicest pre/pros and multi-channel amps and even a couple receiver/processors. None, and I repeat, none are very good with 2-channel performance if you have high demands. All, and I repeat all, were perfectly acceptable to my wifes standards for music. But she was able to tell the differences immediately between doing it the best way for 2-channel and just having 2-channel available.

You have a good 2 channel system right now. If your budget is limited, it may be difficult to have a combined 2 channel/HT system that will match your current 2 channel performance. In this case, it may be best to get a modest receiver for dedicated HT duty. It really depends on your budget as to whether you can find a processor that won't degrade your current 2 channel performance.

Many many 2 channel stereo system owners come to this junction. You do have options, yes. And yet, you might want to maximize your options - but it depends on you variables.
What speakers are you using? Can you get a matching center? If not, I strongly urge against mix-matching any other brands for center channels (even rears, ideally), with your mains! You want matching speakers across to build a coherent, hi-fidelity, believealbe soundstage, that dissapears. Anything else won't do here.
That said, you can still get good fidelity results by simply adding an outboard AV pro/pro and a subwoofer to your system, for very dynamic DD/DTS sound from your system! In this case, something like the Acurus Act 3 DD/dTS 5.1 channel pre would fit nicely. (you can get em from $300-450 on the net, used).
If you add a pre, you will simply loop the av pre/pro into an auxilary or spare (direct?, tape-in?) input on your 2 channel pre - then connect digital and analog connections from DVD, satalite, vcr, etc, into your av pre - everything else stays connected like it is now. This is the easiest way to get into movie processing with quality.
You can then add other amps for center or rears if you add.
Another option is to keep your 2 channel system separate from your multi-channel HT system. You'd then have a matching speaker system dedicated and maximized to play back movie soundtracks in your home! Not knowing your existing speaker system, this is my recommendation.
There are a great many "wall mounted" dappolito configured (woofer, tweeter, woofer), more controlled dispersion "movie oriented" speakers out there, wich will offer you very good quality sound for Soundtracks - which will allow you to get the speakers off the floor, look cleaner and more ergonomical, and you have more placement flexibilitiy, likely.
So, you do have optionsl. Yes, you can loop a receiver into your system too, but you'll have less power, refinement overall, etc.
One thing, the new budget receivers offer some very very good EQ/Room DSP correction, to greatly help acoustics! So, if you have some very efficient speakers, this might be the way to go (adding a receiver, and separated speaker system).
Hope this helps.
So, what is your room/system like, completely?

I've a fairly decent 2 channel setup (Musical Fidelity 308 Integrated fed by a Benchmark DAC1). The 308 splits duty with my home theater as it has a passthrough input that bysteps its preamp. That said, it's paired with another decent 7 channel amp, all fed by a pre/pro processor for home theater duty. It's fairly seamless and I've no complaints.

Might not be the answer you're looking for as you have separate preamp and amplifier already, but a good integrated 2 channel with this type of passthrough helps solve the problem.