Check this out: http://www.mpgh.net/forum/showthread.php?t=1087642
Switch the GTX750Ti to a GTX960 and get a SSD.
Googled quickly and a lot of german people seem to rate http://geizhals.de/?m=1 as the best site for buying PC parts. If you're able to use geizhals you should be able to purchase my entry level rig with GPU upgraded to a GTX960 and SSD. Everything should cost something at around 600€(I counted really quickly could be wrong). Leaving you 100€ if you want to purchase legal copy of windows (Althought you can download it quite simply from sites I can't list and it's a method I can't recommend 8D.)
If you have no experience in building PCs I'd recommend watching LinusTechTips videos from youtube, this for an example is a really detailed build guide:
If you spend little time focusing on guides the building itself shouldn't be a problem at all.
EDIT: For your use 750Ti should be more than fine, so you don't need to purchase better GPU. Althought it's such a cheap upgrade that I would still go for it, since graphics of f2p games are ramping up. If you can fit the 960 definetly go for it, if you can't 750Ti should be more than fine for your use.