Probably should wait a bit:
http://buyersguide.macrumors.com/#iPod_Touch
The problem with the iPhone 3GS (which I still use) is that it isn't an A4 or A5 chip-it is the last of their old processors. The latest iPod Touches, iPhone 4, and both iPads are all A4s or A5s.
I really don't expect future iOS releases past 5 to support anything less than an A4. Actually rather surprised iOS5 supports my iPhone 3GS