There's a discussion somewhere (probably in the 100+ page main thread) about using Market Helper to install incompatible apps. At the time I reported that it wasn't working for me.
I since came across a video on youtube about it, and the poster had a comment that seemed to make all the difference "you must have location services enabled for it to work". I enabled location services, and sure enough it is working now.
NOTE: In his video, rather than downloading apps via the play store on the S7800, he "pushed" them to his S7800 from the play store on a PC. This is what I have just done as well, and it allowed me to install Mass Effect and Dead Space. I haven't tried doing it directly through the play store on the device, not sure if that works or not.
EDIT: Still doesn't work perfectly though... tried to install Wildbood and Backstab this way, and received an error while trying to download.