bigmoguls

I have a Peugeot Triathlon with 52/42 front crank and 21/19/17/16/15/14/13 rear cassette, Shimano 105 component set.

This gives me 10 effective gear ratios and speed range from 14.9mph to 29.7mph (roughly) at 95rpm.

If I switch to a 50/34 crank, I'll have 12 effective gear ratios and speed range of 12mph to 28.6mph at 95rpm.

What do I need to know to get a 50/34 crank to fit this bike?

Option 2 is a whole new bike.

bikemig 

You might want to consider a cross type compact (say 46-36) as this should give you better in between gears with 7 on the back. The big jump in a 50-34 compact tends to work better when you have 9 or more gears on the back. The peugeot triathlon is a newer pug; if it has an english threaded bottom bracket (and I believe that the newer peugeots did), this will be a pretty straight forward upgrade.

Drew Eckhardt 

Get a triple crank if you want more range. 50-40-30 x 13-21 was my favorite in th 8 cog era.

You do NOT want a compact crank.

After moving on to 9 cogs and wearing out bottom bracket + chain ring I noticed that 50-34 x 13-14-15-16-17-18-19-21-23 provided the same range and spacing, and 2 rings are better than 3....

Huge mistake. Instead of spending most of my time on my middle ring I was bouncing back and forth. Dropping below 16 MPH up-hill I'd shift from 50x21 to 34x15 with a five cog change. Speeding back up beyond 18 MPH I'd shift from 34x14 to 50x19. That could happen 10 times a mile on a road which wasn't dead flat.

Switched back to 50-39-30 x 14-23/13-26/12-23 10 cogs and it's immensely better. 39x14 is a nice 20 MPH cruising gear. 39x23 is a nice 10 MPH climbing gear. If I need to scoot a little 39x14 is good for a 29 MPH sprint. The middle ring splits the difference between where big and small are on a double, so the small and large cogs run relatively silently making avoiding them less important.

Velo Dog

I have 46-36-26 triples on both my road bikes, and it's perfect for an old slow guy who lives in the Sierra Nevada. If I ever have to replace one, I'll probably try about a 48-34 double, but just because I want to see what it's like.
FWIW, I've found the alleged shifting difficulties with a triple to be imaginary. How smart do you have to be to shift a front derailleur?

ThermionicScott 

+1 to the above. Most of my riding is in the 60-85" range, so I would constantly be shifting the front and subsequently shifting from one end of the cassette to the other on a 50/34 + 13-21 setup. It only looks good on paper.

It's kind of a moot point, because 13-21 "J" cassettes are hard to come by unless you have a stash. Consider switching your small chainring to a 39T and getting a 13-14-15-17-19-21-23 "I" cassette (which are plentiful), for a low gear that compares nicely with your proposed 34/21.