You can still get the 4.0 update with the 2nd gen iPod, you just can't multitask, which isn't even a big deal because you can just jailbreak the touch if its that big of a deal for you.
Doesn't really add anything new though, so you might as well go with the 2nd gen with more memory. You think that 8 GB will be enough but in reality you are not getting 8GB of space, you are getting like 6 GB. about 2 GB is reserved for the OS and drivers for the touchscreen and such. If your going to spend the money might as well full proof yourself for a couple years, 6GB's is nothing for an iPod. Unless you are just buying it for the internet, get something with more space. Here is an example.
I have the 16GB iPhone 3G S, which is actually 14.3 GB.
Songs - 116 (819.6MB)
Video's - 4 (3 movies, 1 TV show) (5.03GB)
Photos/Video's -74 photos, 12 videos (366.8MB)
Apps - 99 But 21 come pre-loaded with my 3G S, so I've downloaded 78 additional Apps. (3.09 GB)
Other - Text msg's and phone data I'm guessing? (332.1MB)
Free Space - 4.68GB