OK, so I've decided I'm ready to drop a bit of dough on building up the nicest, fairly single-purpose road ride as makes sense for me and my riding type.

About me: Mid-30's, 5'11", 160lbs, living in relatively-flat Boston. I have many bikes already, but I've always been more of a "bike culture" and bike mechanics person than a serious or competitive rider. I consider myself a reasonably strong rider, with a 10-mile every-day commute that I ordinarily do at about 20mph in Boston traffic on my 1983 Schwinn/Giant in 52/16 fixed-gear (brakes, not brakeless!) I like to tinker, and am completely comfortable building up a bike from its barest pieces, and own all my own tools to do so (but no welding...yet.)

My issues: I've been finding that on relatively long road rides (40+ miles), my right knee will get sore and a bit stiff the next day. This happens equally on the fixed-gear or on my geared '01 Kona Yee-Ha running 52/42/32 triple with 11-24 in back. I will go all the way down to 32/24 to get up long hills when I get tired, but usually I'm standing. Maybe it's that I'm pounding too high gears most of the time and I ride fixed too much, or maybe it's just because I'm getting old.

What I think I'm looking for: I suspect building up a proper, light, modern road bike might help. I'd like to get the best bang-for-my-buck, but I'm not gonna be dirt cheap about it like I usually am with my other bikes. I imagine I ought to get a professional bike fit. I have plenty of other bikes, so this bike will be used ONLY for long road rides where touring capacity and durability is not an issue (I've got the Kona and a Burley Rumba tandem for that) and theft is not an issue (I've got Schwinnie for that).

Used stuff is fine, and gives me a warm fuzzy feeling, but I suspect there may not be any real bargains to be found in the used market for this particular kind of ride. I was thinking of building up a Chinese carbon frame like www.flyxii.com , but I'm noticing that with some of the online bike outlets like bikesdirect.com, I can probably do just-as or nearly-as well pricewise and probably get a more consistent-quality product out of it. I've been reading very good things about the Cannondale CAAD9 and CAAD10 frames, so maybe that could get me a bit more bang for the buck than going with full carbon? I do all my own bike mechanic work (with occasional collaboration from the other guys at the bike coop I volunteer at) and intend to continue doing so, and I don't really buy the idea that, for example, properly-tuned Ultegra will be noticeably different from properly-tuned 105. I imagine lighter wheels, on the other hand, will make a huge difference because of rotational weight, and I might be light enough that the durability of lighter wheels isn't much of a problem. What about these new style semi-aero frames like this? The bike culture guy in me prefers the idea of titanium (just seems more soulful and is undoubtedly more durable) over aluminum or carbon, and as much as I love steel, I have plenty of that already and I want something "different". I'm understanding that similar ride characteristics can be squeezed out of any material, some more easily and more lightly than others -- but I just don't know what I should really be looking for here. I imagine these questions may be very personal and I could do well by demo-ing a bunch of bikes of different types at something like Giant Demo Day.

Any suggestions?