If it is your PC http://www.acer.com/ac/en/MM/content/model/DT.SRQST.039, it looks like the micro ATX format.
For the PSU, open the case to check the PSU dimensions, and compared with the info from the link, just make sure the PSU will fit or not.
List of computer PSU form factors
http://silverstonetek.com/techtalk_cont.php?area=en&tid=10055
For the GPU, you need to open the case too, make the measurement to see the GPU will physically fit into htat case or not. You may find the GPU dimensions from GPU manufacturer site. Like
http://www.adventcomputers.co.uk/product-support/how-upgrade-your-desktop-pc%E2%80%99s-graphics-card
Yes, the 400W/450W PSU can handle your PC with gtx750ti, but if you can find the 500W/600W one for less, just buy it, why not? but make sure buy the good brand PSU, eg. XFX, Seasonic, some of the EVGA/CORSAIR, etc.