There are a couple of places in Ximending, but they can be a bit hard to find. Nova pretty much sucks balls for games. Downstairs in K-Mall there are a couple of good places for console games, and there are a few good stores around Raohe, Tonghua, and Shilin Nightmarkets.
Also, In the mall underneath Civil Boulevard (Taipei City Mall ?), You can get there from underground the railway station, then walk west almost to the end, there are a few shops there there. Some selling old stuff.