Well, I just recently acquired an R40, which has P-M 1.5GHz, and I think it runs fine for my needs, which is occasional gaming (radeon 7500 32mb), programming, and school work. I'm sure 1.7Ghz is significantly faster, but the dollars you need to spend for that increase isn't worth it, at least in my opinion. Depends on your own budget too.
According to the benchmarks i've seen on the net, Pentium M 1.4 has roughly the performance of a Pentium 4-M 2.0Ghz , and the Pentium M 1.7 is slightly better than Pentium 4-M 2.6Ghz. (From intel's website anyhow)
Right now, Pentium M 1.5 is only a few bucks more ($30) or so than Pentium M 1.4 (although notebook manufacturers seem to hike up the price alot more), but a 1.7 chip is about twice the price as the 1.4. So dollar for dollar, I'd go for a 1.5 or 1.6 if you are budget conscious.