Clark -
One aside to Woodie's comments.
DCC decoders (the little circuit board inside each loco) are available for as low as $20 each (or even less). I've seen them at $12 each - though that was in packs of 10. These are generally as good as any, just maybe controlling fewer "features" like fancy lighting effects for your diesels.
There are also DCC controller systems available for around $80-100 (Bachmann, Atlas), though in that case I'd urge you NOT to go with the lower-end, but rather with a mid-range system for $250-$300 (MRC Prodigy Advanced, Lenz System 100, etc.)
Having said all that, DCC decoders WITH SOUND start at $70 (one by MRC for diesel or steam) and go up. Soundtraxx is often viewed as top of the line and goes from $120 upward. tonystrains.com and wiringfordcc.com are two good resources to check out for more info.
If it were me, I'd go DCC (I can say that with surety - I did). I did it at first just to simplify running (no more need for blocks, vastly reduced wiring, ability to do things you just can't do with regular DC like run through reverse loops without flipping any polarity), but quickly found that being able to add on-board sound was a huge plus. I also happen to like that my layout is hooked to my computer and I can run the whole thing off the screen if I wish...
Kris