Yea there are sales in the shopping centres. things like Xboxs though don't really change in price.
I recommend using ceneo.pl to find a good deal. It's a price comparison website and it'll search through. And personally, get the 1TB version. Games like GTA, the Witcher are like 60GB now, so you won't be long filling up on the standard 500GB version.
Another good one is skapiec.pl - it is sometimes so, that some shops are not present in Ceneo, but they are present in Skąpiec and they may have lower prices.
Ceneo is pretty good but I find that Allegro is usually better, especially for things such as Xboxes and games. The only thing to remember with Allegro is to always and only pay with a credit card, as the support from Allegro if the seller decides to simply keep your money and send you nothing (or sends you very bad fakes of what you bought) is pretty much sod all.
