I love having my Treo 650 "office in my pocket" and so do many of my real estate clients (I'm a marketing consultant and writer for agents and brokers).
Several of my techie RE clients use the Treo 650, and they swear by the Top Producer product for the Palm. I've tried the Act! product and didn't care for it, myself.
The best place to try out the freebie and the paid programs? PalmGear.com. Unlike some of the other vendors, I've never had a problem with the checkout, or any resolution with them. They also keep a record of everything you buy online, so you can re-download in case you misplace the original file, or want to update to the latest version.
I will advise that a backup to SD card is essential. When you are out and something (goodness forbids) happens to your Treo and you have to hard reset (it's only happened to me once) -- you can recover in under 5 minutes if you have this SD backup on your expansion card. I've tested and used both BackupBuddy and Resco's Explorer products with success. BackupBuddy is faster and easier, but Explorer is more flexible and will allow for partial backups and partial retrievals, which is nice.
If you use the headset, (or the speakerphone) as someone mentioned in a related post, you can multi-task on the little guy. With a program like VolumeCare (which boosts the natural volume level of the speaker and the headset and the handset and the mic) you will enjoy using the Treo more.
One of the most often used programs by my real estate clients (which I understand comes with Supra eKey software) is a loan calculator called LoanPro. In markets that offer the electronic lockboxes, the agents love the Supra eKey.
Personally, I have to use a 2 gig card to keep all my "essential" programs and I tote a spare 1 gig card for my "extras" -- but I'm a geek. I do nearly everything for my business on my Treo when I'm out of the office. I have a bluetooth keyboard that makes it simple to leave my laptop at the office. I have an email program (snappermail) that I highly recommend over the pre-packaged versamail product. I record messages by writing, thumbboarding, typing or by using a program that records voice notes. I can call, email or IM. In my business, communication is the key -- as in yours. The best IM program I've found is Mundu.
Whenever I leave my office, the Treo is my pocket office, communicatons central and entertainment all-in-one. (I listen to music, read ebooks, hear audiobooks and even watch movies on mine!)
I often share more information on this particular item on my hobby site (WickedTreo.com). Yeah, keeping up-to-date on what's new with the Treo and the software has become a full fledged hobby, and the hobby-site is dedicated to helping Real Estate Agents (like everyone here) and writers (like me) use this pocket-sized tool to run the business whenever they step out of the office.
I hope this helps!
Angela