Probably one of the most widely known BT headsets on the market the Jabra BT200. When it the Nokia 3650 first came out this headset was not compatible because it didn't support the Handsfree profile that all Nokia's and many of the newer Series60 phones use for audio transmission. Jabra was offering a free replacement to those that had one of the incompatible models if you called and requested one. That's been some time since that was offered but may still be available by calling Jabra. So if you're buying on eBay check that the BT200 is either updated or specifically labeled as compatible for the Nokia 3650.
Since then lots of headsets have come out on the market.
I'm also a fan of the Jabra BT250, similar to the BT200 but given a make over with gray and black molding. In addition this headset has a battery life rating of twice that of the BT200 at 8 hours.
This headset design is very comfortable to wear for extended periods and unlike some models that appear to hang off you ear this headset actually wraps around your ear. Certainly not the best headset out there but a solid choice for anyone wanting a wireless headset. Not to mention at less than $100 from most retail outlets and even less on eBay it doesn't break the bank.