Unless you are running a lot of software that can use the extra 4th core (e.g. transcoding, raytracing, etc), I'd go with the PII X3. It'll be more overclockable as well, and faster on most apps.
Get the 720. It has a bigger cache, higher clock speed, and more OC headroom. This gives it much more real-world performance, and the updated K10 architecture seems to handle GPU bottlenecks (which you will see in almost every gaming situation with modern CPUs) very well.
Only reason to get the Phenom 9850 is if you need 4 cores, or the computer is being used exclusively for a program/programs that utilize all 4 cores. Almost all applications and games will run faster on the X3 720 due to it having a slightly higher clock speed, more L3 cache and being faster clock for clock (45nm Phenom II is faster than 65nm Phenom at the same clock speed).
You might want to look into getting some cheap DDR3 and an AM3 board with a 720. You can probably find some high-voltage DDR3 being blown out somewhere since the i7 doesn't support much.