Navigating the Osaka train system is wonderful, it's much more easier than Tokyo's. If you have a problem with anything, ask a station attendant, they're good at helping.
I guess I'd ask what it is you want to do in Den Den Town. If you're into retro games, toys, or just bumming around the arcades for an afternoon, it's definitely worth it. Geo-Front has one of the most comprehensive selection of PS2 games I know of, and their other retro game selection is pretty nice too. You might as well check out the Super Potato, but their selection is a lot smaller than the one in Tokyo. If that sound like your bag, go for it.
Of course, Den Den Town is within walking distance of the Namba station, so there's a lot of other places you can wander around that are less nerdy. As mentioned above, Dotonbori a great, lively place to walk around, especially at night time when all the neon is lit up. A lot of famous imagery associated with Osaka runs along this canal.
I think it's worth checking out the Shinsekai district too, which is where Tsuutenkaku broadcasting tower is, another famous landmark from this region. The neighborhood still has a very mid century feel to it (according to Wikipedia, it received very little reconstruction after the war and is still quite poor), and the atmosphere is quite unique (it's Japan though, so I guess the atmosphere's gonna be unique everywhere you go). It's also really easy to get to. Just take the JR line from Namba to Shin-Imamiya, it's only two stops away. Speaking of unique, it's too bad it's not baseball season yet, or I'd tell you to go see a Tigers game.
Universal Studios Japan is close by too, but I think that's more interesting if you have any connection to either of the Universal Studios in the US (It's the only place left where you can still find the Back to the Future ride).
100,000 for a week and a half may or may not be enough, especially if you like to spend money, but as Orionw said, ATMs are in easy supply. They won't all take international cards, so it's probably best to stick the ones inside 7-11's to be sure. Have fun!
I was like, 'ugh, this is all wrong', and then I changed it to black. I think it looks pretty cool, though I haven't fooled around with it yet. I like the new user icons, it makes me feel like I've won something.