i have used ppcs and PDAs for a while now currently i have a ppc smart phone which is basically a meld of mobile phone and PDA its based around the HTC magician which is a generic platform and many UK networks use it as a template for own brand ppcs and the OS is windows 2003 mobile edition and uses an intel Xscale 415mhz processer.
the main thing to check before u buy a PDA is first of all the size and weight. do u intend to fit in a briefcase or pocket? also battery times as some PDAs have a back up battery and if u forgot or are unable to charge it u can lose all ur data and settings and also the possible cost or replacement batteries as this can be a sting and Li ion batteries are not very durable. memory of course is extremely important and check the connectivity options to infrared, blue-tooth and on higher end models Wi Fi and see what expansion and multimedia card slots it has ie for satnav or permanent storage. many models now also have mega pixels cameras but in my experience even these have limited use as a camera. but the main thing is don't get fixed on the price some of the cheaper ones are as good and sometimes are far better and ask in the shop if they have a fully functional one to try out not just the plastic dummies