Hey heemid,
Where in TX you at? I'm in Houston.
Good comments from the above posters. I tend to concur with the folks who advocate something a bit bigger. My cafe project is a '78 XS650, and will weigh quite a bit less than a stock bike when I'm done, probably slightly under 400 lb. Yes, the old XS vibrates because it's a 360 twin, but you get used to it. It's not as bad as lots of folks make out. And hey, leave us not forget, the original cafe bikes were either asshammers (singles) or 360 Brit twins. There are several advantages to building an XS650 cafe:
Yamaha built a kazillion of 'em, so they're still fairly common
Because they're common, they're usually cheap. I know several folks that got theirs for free! I ended up paying about $175 for my project bike.
Parts availability is great, including a wide (and growing) variety of performance and custom parts
Post '73 XS650s have a much improved frame, and as a result they're just plain fun to ride.
They just look cool and sound cool too.
If you really want to stick with something smaller, then you need to ask yourself a few questions, like:
2 stroke or 4 stroke? If a 2-stroker, pretty much the best way to go is an RD350 or RD400. Great little bikes!
Single, twin, or four cylinder? Singles are very cool (my favorite smaller ones are the SR400 and SR500), but they're generally slower than multi-cylinder machines and generally vibrate more. Of the small twins, I'd stick with either the CB350 or CL350, but not the later CB360. It had issues. I learned how to ride on a CL350, and I have fond memories of that machine. 10.5k redline, whoooeeee! The Honda 4s are popular -- just look around here.
How much are you willing to spend, and how much work are you willing to do? (This applies to any project, really, regardless of size or model) Some of the older models, like the Honda 350s, have become quite desirable to old bike collectors, and it's getting hard to find clean examples at decent prices. The rougher the bike, the more you'll have to spend on it, and the more work you'll have to do to get it where you want it. Often one is better off paying more for a clean bike than bringing a junker up to snuff.
Best,
Michael