I use cingular as the provider. I'm not 100% into cingular though...Verizon is another good (prob better) option. Either way, you will need to add an unlimited data plan which is ~$40 on either carrier. Trust me with the unlimited. If you use the data features alot, and dont have unlimited, you'll get hammered. I have seen several bills well over $1000 from people not adding the proper data plan.
A treo 650 will come with all the software you need to sync with your laptop. If you have bluetooth in your laptop, you can sync via that, otherwise the cable is fine. Bluetooth is a SLOOOOW sync, esp if you have alot of data on there as i do. The 650 also comes with a great browser and email client for wireless access to www and email.
I think it is worth every penny, but I'm an IT guy by trade. If you are willing to devote some time to learning the device, it will pay you back many times over. If you dont, you can be like some of my clients, and pay me too much money to setup / fix / maintain your device.