I've built up 4-5 bikes over the past year or so. I figure that since I'm probably gonna be riding for the long-haul, I might as well invest some serious money in a nice ride. I commute by bike and frequently go on leisurely rides (around 40km). Right now my main bike is a Peugeot U0-8 that has most of the parts from a U0-12 that was too small for me. I have it tuned to a T and it's riding really nicely right now. I like the relatively slack geometry - makes a springy, comfy ride.

As far as the upgrade is concerned:

I want to build it up myself from the frame up. I've been thinking about buying a vintage steel frame (70's-80's). However, since I don't really have the means to do a proper, historically accurate restoration, is it stupid to spend $400-700 on a vintage frame (for example: http://www.bikingthings.com/bicycles.html)? Would it be more worth it to put that money into a new frame? I really like vintage frames; the geometry, lugwork, welding give them a beauty that doesn't really hold up in newer bikes.

Also, besides weight, what would I really gain? I'm sure there's a discernible difference but as far as riding the bike is concerned, what benefits would a bike in the price range I stated earlier give me over my U08?