Steve,

For me the key points are fork geometry, the ability to get the saddle set back far enough from the BB, the reach from saddle to bars, and height of the bars relative to the saddle. I also like a low BB and no toe overlap with the front tire or fender. I would take these contact point specs from your existing bike.

It takes some sketching and figuring to turn those numbers into criteria for a new bike. Setback for example can be translated into seat tube angle, but it depends on saddle rail length and position, and the available seat post setback. But given a saddle and a seat post, you can come up with the seat tube angle that will let you attach the saddle in the middle of the rails - this gives you maximum ability to fine-tune as time goes on.

We can't tune the front end or BB drop, so if you know what you want, you just have to buy the correct geometry. Bar position and height can be tuned. One good reason to start with maybe a 9 cm stem is that you can get a longer one or a shorter one if you need it later. Actual reach to the hoods is affected by handlebar reach and width, as well, so some trigonometry is useful here, too.

My plan was to do just that, including the BB height. It seems that what is critical for a good fit is that central "triangle" between the top tube, downtube and seat tube, so if I can get something pretty close to my road bike (and my handlebars are just a scrunch above my seat, so I'm not in a real aggressive riding position), I should be able to dial it in just as well.

Running down all the suggestions folks have made here has helped me clarify what I'm looking for besides the dimensions of that triangle, which pretty much boils down to the ability to put slightly larger tires on it. That Ribble frame that tsl recommended, while clearly a good price point, can only handle 23mm tires (I run 25's on my road bike!), and I'd like to have the option of going as heavy as 28-32 AND be able to mount fenders.

If the Surly's (e.g. the Pacer or Cross Check) have measurements that work they look to be a good choice - I need to compare the geometry to my current bike. My LBS is a Surly dealer and might be able to offer some purchase advice on one of those.

I'm leaning toward buying a new frame rather than poking around looking for something used - I like the ability to be able to purchase a known quantity w/regard to dimensions.

That said, I'm kinda leaning toward the Soma ES. It has the tire capacity that I'm looking for (up to 32mm) relaxed geometry, "classic" profile with a horizontal top-tube, braze-ons for fender and rack attachment, priced within my budget, and the critical dims are so close to my current roadie (a Specialized Secteur) that I should be able to dial it in to fit me nicely. The only significant differences is a slightly higher BB and a little taller SO height which I'd expect with the horizontal top tube:

[table="width: 500, class: grid, align: left"]
[tr]
[td]Dimension[/td]
[td]Current[/td]
[td]Soma ES[/td]
[/tr]
[tr]
[td]Size[/td]
[td]52cm[/td]
[td]52cm[/td]
[/tr]
[tr]
[td]Top Tube[/td]
[td]537mm[/td]
[td]540mm[/td]
[/tr]
[tr]
[td]B-B Drop[/td]
[td]73mm[/td]
[td]70mm[/td]
[/tr]
[tr]
[td]Seat-Tube Angle[/td]
[td]74*[/td]
[td]74*[/td]
[/tr]
[tr]
[td]Head-Tube Angle[/td]
[td]72*[/td]
[td]73*[/td]
[/tr]
[tr]
[td]Stand-Over Height[/td]
[td]756mm[/td]
[td]774mm[/td]
[/tr]
[tr]
[td]Reach[/td]
[td]378mm[/td]
[td]380mm[/td]
[/tr]
[/table]

Not real excited about the color ("Capuccino") but it is what it is.

I'm a bit puzzled as to what fork I should marry up to this frame - None of the forks on Soma's site seem to be made to match this frame, so I'll probably have to call them about this.