Sadly none of them have a pcie x16 slot so any modern gpu is out of the question. The medium sized one isn't clear. There is a mini tower, desktop and small form factor. ftp://ftp.dell.com/Manuals/all-products/esuprt_desktop/esuprt_optiplex_desktop/optiplex-gx520_user%27s%20guide_en-us.pdf
Although I'd suggest not even wasting money on such an old system. The best you could do is 8-9yr old games, the time it was made.