The 3rd gen 8GB is just a 2nd gen inside, so don't buy that one.
If you want the real upgrades you have to get 4th gen, and they are big upgrades. I have the 4th gen and it has 256mb ram, a-4 processor, camera with video, front camera for facetime, built in mic etc. 2nd gen has.... none of this and only 64mb of ram if I am correct, I may be wrong, I don't own a 2nd gen, but its nothing like the 4th gen trust me.
Also please get 32GB or higher, you really can't live with 8GB especially when the software takes up 2 or more gigs of that.
Rumor has it that the apple event is on october 4th this year so I would wait till that before buying any iPod, although this year is not supposed to be a big upgrade year for the iPod touch, what else can they add to it before it becomes the iPhone??? I think they are only making a white one this year with very slight upgrades.
The 4th gen is totally worth it, its one of the best products I own, its basically a smartphone without the super high bills. There are apps that do free texting and whistle lets you make phone calls under 20 min for free. Of course it has to be connected to wifi to do this but it really becomes a phone when its connected to wifi. Some of the games they have out now are pretty engaging, there are a lot of shovelware games but there are good ones on the app store too.